Explanation/Reference:
Because the Reference Monitor is responsible for access control to the objects by the subjects it compares the security labels of a subject and an object.
According to the OIG: The reference monitor is an access control concept referring to an abstract machine that mediates all accesses to objects by subjects based on information in an access control database. The reference monitor must mediate all access, be protected from modification, be verifiable as correct, and must always be invoked. The reference monitor, in accordance with the security policy, controls the checks that are made in the access control database.
The following are incorrect:
Validation Module. A Validation Module is typically found in application source code and is used to validate data being inputted.
Clearance Check. Is a distractor, there is no such thing other than what someone would do when checking if someone is authorized to access a secure facility.
Security Module. Is typically a general purpose module that prerforms a variety of security related functions.
References:
OIG CBK, Security Architecture and Design (page 324)
AIO, 4th Edition, Security Architecture and Design, pp 328-328.
Wikipedia - http://en.wikipedia.org/wiki/Reference_monitor

Security testing and trusted distribution are required for Life-Cycle Assurance.
The following answers are incorrect:
System Architecture and Design specification. Is incorrect because System Architecture is not requried for Life-Cycle Assurance.
Security Testing and Covert Channel Analysis. Is incorrect because Covert Channel Analysis is not requried for Life-Cycle Assurance.
Configuration Management and Trusted Facility Management. Is incorrect because Trusted Facility Management. is not requried for Life-Cycle Assurance.

Section: Security Operation Adimnistration
Explanation/Reference:
This is the most serious risk of inadequate systems development life cycle methodolgy.
The following answers are incorrect because :
The project will be completed late is incorrect as it is not most devastating as the above answer.
The project will exceed the cost estimates is also incorrect when compared to the above correct answer.
The project will be incompatible with existing systems is also incorrect when compared to the above correct answer.
Reference: Information Systems Audit and Control Association, Certified Information Systems Auditor 2002 review manual, chapter 6: Business Application System Development, Acquisition, Implementation and Maintenance (page 290).

Explanation/Reference:
The original TACACS, developed in the early ARPANet days, had very limited functionality and used the UDP transport. In the early 1990s, the protocol was extended to include additional functionality and the transport changed to TCP.
TACACS is defined in RFC 1492, and uses (either TCP or UDP) port 49 by default. TACACS allows a client to accept a username and password and send a query to a TACACS authentication server, sometimes called a TACACS daemon or simply TACACSD. TACACSD uses TCP and usually runs on port
49. It would determine whether to accept or deny the authentication request and send a response back.
TACACS+
TACACS+ and RADIUS have generally replaced TACACS and XTACACS in more recently built or updated networks. TACACS+ is an entirely new protocol and is not compatible with TACACS or XTACACS. TACACS+ uses the Transmission Control Protocol (TCP) and RADIUS uses the User Datagram Protocol (UDP). Since TCP is connection oriented protocol, TACACS+ does not have to implement transmission control. RADIUS, however, does have to detect and correct transmission errors like packet loss, timeout etc. since it rides on UDP which is connectionless.
RADIUS encrypts only the users' password as it travels from the RADIUS client to RADIUS server. All other information such as the username, authorization, accounting are transmitted in clear text. Therefore it is vulnerable to different types of attacks. TACACS+ encrypts all the information mentioned above and therefore does not have the vulnerabilities present in the RADIUS protocol.
RADIUS and TACACS + are client/ server protocols, which means the server portion cannot send unsolicited commands to the client portion. The server portion can only speak when spoken to. Diameter is a peer-based protocol that allows either end to initiate communication. This functionality allows the Diameter server to send a message to the access server to request the user to provide another authentication credential if she is attempting to access a secure resource.
Reference(s) used for this question:
http://en.wikipedia.org/wiki/TACACS
and
Harris, Shon (2012-10-18). CISSP All-in-One Exam Guide, 6th Edition (p. 239). McGraw-Hill. Kindle Edition.