The correct answer is "Auser professing an identity to the system". A user presents an ID to the system as identification. Answer a is incorrect because presenting an ID is not an authentication act. Answer "Auser providing a password to the system" is incorrect because a password is an authentication mechanism. Answer "Auser providing a shared secret to the system" is incorrect because it refers to cryptography or authentication.

The correct answer is "TACACS+ employs tokens for two-factor, dynamic password authentication".
TACACS employs a user ID and static password and does not support prompting for password change or the use of dynamic password tokens.

UDP (User Datagram Protocol) is a communications method (protocol) that offers a limited amount of service when messages are exchanged between computers in a network that uses the Internet Protocol (IP). UDP is an alternative to the Transmission Control Protocol (TCP) and, together with IP, is sometimes referred to as UDP/IP. Like the Transmission Control Protocol, UDP uses the Internet Protocol to actually get a data unit (called a datagram) from one computer to another. Unlike TCP, however, UDP does not provide the service of dividing a message into packets (datagrams) and reassembling it at the other end. Specifically, UDP doesn't provide sequencing of the packets that the data arrives in. UDP is implemented at the Transport layer of the TCP/IP protocol model.

Multiprotocol Label Switching (MPLS) is a WAN technology that requires the first router in the path to determine the full path the packet will travel, removing the need for other routers in the path to make independent determinations. MPLS works by adding a label to each packet at the ingress router, which indicates the forwarding equivalence class (FEC) of the packet. The FEC is a group of packets that share the same destination and quality of service (QoS) requirements. The label is then used by the intermediate routers to forward the packet along a predetermined label-switched path (LSP), without inspecting the packet header or performing routing lookups. The label is removed at the egress router, and the packet is delivered to the destination. MPLS can improve the performance, scalability, and efficiency of WAN networks, as well as support multiple protocols and services12. References: CISSP CBK, Fifth Edition, Chapter 4, page
343; CISSP Practice Exam - FREE 20 Questions and Answers, Question 15.