Another form of attack is called the distributed denial of service (DDOS). A distributed denial of service occurs when the attacker compromises several systems and uses them as launching platforms against on or more victims. - Ed Tittle CISSP Study Guide (sybex) pg

The question mentions clearly "operating together". Which means the BEST answer is Dual Control.
Two mechanisms necessary to implement high integrity environments where separation of duties is paramount are dual control or split knowledge.
Dual control enforces the concept of keeping a duo responsible for an activity. It requires more than one employee available to perform a task. It utilizes two or more separate entities (usually persons), operating together, to protect sensitive functions or information.
Whenever the dual control feature is limited to something you know., it is often called split knowledge (such as part of the password, cryptographic keys etc.) Split knowledge is the unique "what each must bring" and joined together when implementing dual control.
To illustrate, let say you have a box containing petty cash is secured by one combination lock and one keyed lock. One employee is given the combination to the combo lock and another employee has possession of the correct key to the keyed lock. In order to get the cash out of the box both employees must be present at the cash box at the same time. One cannot open the box without the other. This is the aspect of dual control. On the other hand, split knowledge is exemplified here by the different objects (the combination to the combo lock and the correct physical key), both of which are unique and necessary, that each brings to the meeting.
This is typically used in high value transactions / activities (as per the organizations risk appetite) such as:
Approving a high value transaction using a special user account, where the password of this user account is split into two and managed by two different staff. Both staff should be present to enter the password for a high value transaction. This is often combined with the separation of duties principle. In this case, the posting of the transaction would have been performed by another staff. This leads to a situation where collusion of at least 3 people are required to make a fraud transaction which is of high value.
Payment Card and PIN printing is separated by SOD principles. Now the organization can even enhance the control mechanism by implementing dual control / split knowledge. The card printing activity can be modified to require two staff to key in the passwords for initiating the printing process. Similarly, PIN printing authentication can also be made to be implemented with dual control. Many Host Security modules (HSM) comes with built in controls for dual controls where physical keys are required to initiate the PIN printing process.
Managing encryption keys is another key area where dual control / split knowledge to be implemented.
PCI DSS defines Dual Control as below. This is more from a cryptographic perspective, still useful:
Dual Control: Process of using two or more separate entities (usually persons) operating in concert to protect sensitive functions or information. Both entities are equally responsible for the physical protection of materials involved in vulnerable transactions. No single person is permitted to access or use the materials (for example, the cryptographic key). For manual key generation, conveyance, loading, storage, and retrieval, dual control requires dividing knowledge of the key among the entities. (See also Split Knowledge).
Split knowledge: Condition in which two or more entities separately have key components that individually convey no knowledge of the resultant cryptographic key.
It is key for information security professionals to understand the differences between Dual Control and Separation of Duties. Both complement each other, but are not the same.
The following were incorrect answers:
Segregation of Duties address the splitting of various functions within a process to different users so that it will not create an opportunity for a single user to perform conflicting tasks.
For example, the participation of two or more persons in a transaction creates a system of checks and balances and reduces the possibility of fraud considerably. So it is important for an organization to ensure that all tasks within a process has adequate separation.
Let us look at some use cases of segregation of duties
A person handling cash should not post to the accounting records A loan officer should not disburse loan proceeds for loans they approved Those who have authority to sign cheques should not reconcile the bank accounts The credit card printing personal should not print the credit card PINs Customer address changes must be verified by a second employee before the change can be activated.
In situations where the separation of duties are not possible, because of lack of staff, the senior management should set up additional measure to offset the lack of adequate controls. To summarise, Segregation of Duties is about Separating the conflicting duties to reduce fraud in an end to end function.
Need To Know (NTK): The term "need to know", when used by government and other organizations (particularly those related to the military), describes the restriction of data which is considered very sensitive. Under need-to-know restrictions, even if one has all the necessary official approvals (such as a security clearance) to access certain information, one would not be given access to such information, unless one has a specific need to know; that is, access to the information must be necessary for the conduct of one's official duties. As with most security mechanisms, the aim is to make it difficult for unauthorized access to occur, without inconveniencing legitimate access. Need-toknow also aims to discourage "browsing" of sensitive material by limiting access to the smallest possible number of people.
EXAM TIP: HOW TO DECIPHER THIS QUESTION
First, you probably nototiced that both Separation of Duties and Segregation of Duties are
synonymous with each others. This means they are not the BEST answers for sure. That was an
easy first step.
For the exam remember:
Separation of Duties is synonymous with Segregation of Duties
Dual Control is synonymous with Split Knowledge
Reference(s) used for this question:
Hernandez CISSP, Steven (2012-12-21). Official (ISC)2 Guide to the CISSP CBK, Third Edition
((ISC)2 Press) (Kindle Locations 16048-16078). Auerbach Publications. Kindle Edition.
and
http://www.ciso.in/dual-control-or-segregation-of-duties/

Radius, TACACS and DIAMETER are classified as authentication, authorization,
and accounting (AAA) servers.
Source: TIPTON, Harold F. & KRAUSE, MICKI, Information Security Management Handbook, 4th
Edition, Volume 2, 2001, CRC Press, NY, Page 33
also see:
The term "AAA" is often used, describing cornerstone concepts [of the AIC triad] Authentication,
Authorization, and Accountability. Left out of the AAA acronym is Identification which is required before the three "A's" can follow. Identity is a claim, Authentication proves an identity, Authorization describes the action you can perform on a system once you have been identified and authenticated, and accountability holds users accountable for their actions.
Reference: CISSP Study Guide, Conrad Misenar, Feldman p. 10-11, (c) 2010 Elsevier.

Answer c is an example of a system-specific policy, in this case the router's access control lists. The other three answers are examples of issue-specific policy, as defined by NIST. Issue-specific policies are similar to program policies, in that they are not technically focused. While program policy is traditionally more general and strategic (the organization's computer security program, for example), issue-specific policy is a nontechnical policy addressing a single or specific issue of concern to the organization, such as the procedural guidelines for checking disks brought to work or e-mail privacy concerns. System-specific policy is technically focused and addresses only one computer system or device type. Source: National Institute of Standards and Technology, An Introduction to Computer Security: The NIST Handbook Special Publication 800-12.