Which protocol is commonly used for secure transactions on the web?

The correct choice is the protocol commonly used for secure transactions on the web, which is HTTPS. HTTPS stands for Hypertext Transfer Protocol Secure. It is an extension of HTTP (Hypertext Transfer Protocol) that incorporates security measures to protect data exchanged between a user's web browser and a server. This security is achieved through the use of Transport Layer Security (TLS) or, historically, Secure Sockets Layer (SSL) protocols.

When a website is accessed via HTTPS, it means that the data transmitted is encrypted, which helps defend against eavesdropping, tampering, and man-in-the-middle attacks. This is particularly important for transactions that involve sensitive information, such as online banking, shopping, and signing into accounts.

In contrast, HTTP does not provide encryption, making it less secure for transmitting sensitive information. While FTP (File Transfer Protocol) and SFTP (SSH File Transfer Protocol) are used for transferring files, they are not primarily focused on securing web transactions. Thus, HTTPS is the correct answer as it directly relates to secure web transactions.