Which protocol should be used to transfer large files if the firewall permits TCP ports 20 and 21?

FTP (File Transfer Protocol) is the correct choice because it uses TCP ports 20 (for data transfer) and 21 (for control commands) to facilitate the transfer of files, including large ones. The question explicitly states that these ports are permitted by the firewall, making FTP a viable and common solution. SMTP (Simple Mail Transfer Protocol) is used for sending email and typically operates on TCP port 25, not for general file transfer. REST API (Representational State Transfer Application Programming Interface) is an architectural style for web services, not a file transfer protocol itself, and doesn't inherently use ports 20/21 for file transfer. TFTP (Trivial File Transfer Protocol) is a simpler, connectionless protocol that uses UDP port 69 and is generally used for small, quick transfers, not large files, and is not specified to use TCP ports 20/21.