Which protocol is commonly used for secure file transfers?

SFTP, or Secure File Transfer Protocol, is commonly used for secure file transfers because it provides a secure channel over an unsecured network. It leverages the SSH (Secure Shell) protocol to encrypt the data being transferred, ensuring the privacy and integrity of the files in transit. This means that any credentials, file contents, and commands sent across the network are secured against eavesdropping and tampering.

In contrast, FTP (File Transfer Protocol) does not inherently include robust security features, which can expose files to interception or modification during transfer. HTTP (Hypertext Transfer Protocol) is primarily used for transferring web pages and does not natively support file transfer with security; however, HTTPS (its secure version) does provide security for web data. SMTP (Simple Mail Transfer Protocol) is mainly used for sending emails, not for file transfers, and does not include inherent security features for file transfer.

SFTP's use of encryption and authentication makes it the preferred choice for secure file transfers in environments where data sensitivity is critical, making it the correct answer for this question.