Which protocol is associated with TCP port 21?

The protocol associated with TCP port 21 is FTP Control. FTP, or File Transfer Protocol, is primarily used for transferring files over a network and operates in two modes: active and passive. The control connection, which is established over port 21, is responsible for sending commands and responses that manage the session and file transfers. This port is used to initiate the connection, allowing clients to issue commands such as 'send', 'receive', 'list', etc., to the FTP server.

The other options relate to different protocols that use distinct ports; for instance, SSH typically communicates over port 22, Telnet over port 23 for unencrypted communication, and HTTP utilizes port 80 for web traffic. These are separate protocols serving different purposes in network communication, clearly distinguishing FTP's function on port 21.