Which of the following ports is used to securely transfer files between remote UNIX systems?

Port 21 is used by FTP (File Transfer Protocol), which transmits data and credentials in plaintext and provides no encryption.

SFTP (SSH File Transfer Protocol) and SCP (Secure Copy Protocol) both operate over SSH on port 22, providing fully encrypted and authenticated file transfers between UNIX systems. Unlike FTP, they protect both credentials and file contents in transit, making port 22 the correct answer for secure UNIX-to-UNIX file transfer.

Port 69 is used by TFTP (Trivial File Transfer Protocol), which has no authentication or encryption and is primarily used for network booting scenarios.

Port 445 is used by SMB/CIFS for Windows file sharing and is not associated with file transfers between UNIX systems.