Which of the following is a TACACS+ message sent from the AAA client to the AAA server?

TACACS+ (Terminal Access Controller Access-Control System Plus) is a protocol used for network authentication and provides a mechanism for clients (like routers or switches) to communicate with a server for user authentication, authorization, and accounting.

In the context of TACACS+, when the AAA client initiates communication with the AAA server, it sends messages to establish the session and perform operations. The "START" message indicates the initiation of a session from the client to the server. This is the first step in the TACACS+ communication process, allowing the server to know that it should prepare to receive further authentication requests and process them accordingly.

The other terms, while related to various functions within broader authentication protocols, do not specifically denote messages within the TACACS+ framework in the same manner as "START." For instance, "AUTH" could denote an authorization process, "INIT" is more general and does not specifically represent a TACACS+ command, and "REQUEST" is a broad term that doesn't match the specific communication pattern of the protocol. Thus, "START" is the clear and correct choice as it accurately represents a message sent from the AAA client to the AAA server in the TACACS+ context.