What method in SIP terminology is defined as a message that terminates a session by either endpoint?

The term that defines a message which terminates a session by either endpoint is indeed 'C. BYE'. In the context of the Session Initiation Protocol (SIP), the BYE message is specifically designed to indicate that one party wishes to end the session they established with the other party.

When a BYE message is sent, it signifies the intent to terminate the ongoing communication, effectively nullifying the session established earlier through SIP. This is a fundamental aspect of SIP as it allows for orderly and clean session completion, giving both endpoints clear communication regarding the termination.

In contrast, the other options serve different purposes within SIP. OPTIONS is used to query the capabilities of a user agent or server and to determine the communication options available. INVITE is employed to initiate a session between endpoints, allowing users to join the conversation. REGISTER is used for a user agent to register with a SIP server, facilitating the identification and location of endpoints for future communications.

Understanding the distinct roles of these messages is crucial for managing and terminating sessions within the architecture of SIP effectively.