Which protocol uses TCP port 20 for its operations?

The correct choice refers to the File Transfer Protocol (FTP) Data, which utilizes TCP port 20 specifically for transferring data between client and server after a connection is established.

In the context of FTP, two separate ports are used: port 21 is primarily for command and control messages and establishes the session, while port 20 is utilized to handle the actual transfer of files in active mode. When a client initiates a file transfer, after the session is set up on port 21, the data connection is made on port 20. This distinction is crucial for understanding how FTP operates in terms of its command structure and data handling.

Understanding this division of ports helps clarify the operational dynamics of FTP and is a critical aspect of TCP/IP networking, which is a foundational component of CCNA studies.