TCP port 21 is used by:

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Multiple Choice

TCP port 21 is used by:

Explanation:
TCP port 21 is the standard control channel for FTP. This is where the FTP client connects to send commands (like login, list, retrieve, store) and receive responses from the server. The actual file transfers use a separate data channel, which can be port 20 in active FTP or a negotiated range in passive FTP, not port 21. The other options map to different services: SSH uses port 22, DNS uses port 53, and SFTP runs over SSH (also port 22).

TCP port 21 is the standard control channel for FTP. This is where the FTP client connects to send commands (like login, list, retrieve, store) and receive responses from the server. The actual file transfers use a separate data channel, which can be port 20 in active FTP or a negotiated range in passive FTP, not port 21. The other options map to different services: SSH uses port 22, DNS uses port 53, and SFTP runs over SSH (also port 22).

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