What is the function of TCP port 21?

TCP port 21 is specifically designated for the File Transfer Protocol (FTP) command control, which includes the management of FTP sessions. When a client wants to connect to an FTP server to transfer files, it establishes a connection to port 21. This port is used for sending commands and controlling the session, such as logging in, sending commands to navigate directories, or to list files.

Data transfer actually occurs on a different port, typically port 20 for active mode FTP or a randomly assigned port for passive mode, separating command and data channels. Given that option B accurately reflects the primary role of port 21 in handling FTP control commands, it is the correct choice.

Other options refer to functions associated with different protocols or ports and do not accurately describe the specific purpose of TCP port 21.