Which protocol operates on ports 20 and 21 and is primarily used for transferring files?

File Transfer Protocol (FTP) is the correct answer because it is specifically designed for transferring files over a network. It operates using two ports: port 21 is used for the control connection, where commands are sent and responses are received, while port 20 is used for the data connection, facilitating the actual transfer of files between the client and the server.

FTP allows users to upload or download files, making it an essential protocol for file management on servers. This protocol is widely used in web development and system administration for sharing files between systems.

Other protocols listed are not designed for file transfer. Simple Mail Transfer Protocol (SMTP) is used for sending emails, Hypertext Transfer Protocol (HTTP) is for transferring web pages and content over the internet, and Secure Shell (SSH) is primarily a protocol used for secure remote login and command execution on systems, not specifically for file transfer.