Secure Shell (SSH) operates over port 22 and provides a secure method of accessing network services over an unsecured network. When it comes to file transfers, SSH is the protocol that is commonly used to secure them. SSH uses encryption to secure the connection between the client and server, making it the best choice for encrypted file transfers. While FTP (port 20/21) is used for file transfers, it is not inherently secure. Telnet (port 23) is used for remote command-line login and does not support file transfers. Port 110 is used by POP3 for email retrieval and does not involve direct file transfer capabilities.