Which protocol is used for secure data transmission over the web?

HTTPS, or Hypertext Transfer Protocol Secure, is the protocol specifically designed for secure data transmission over the web. It combines HTTP with an additional layer of security provided by Transport Layer Security (TLS) or its predecessor, Secure Sockets Layer (SSL). This secure connection encrypts the data exchanged between a user's browser and the website, making it extremely difficult for unauthorized parties to intercept or alter the information during transmission.

The significance of HTTPS is particularly evident when handling sensitive data such as personal information, credit card numbers, and login credentials. By using HTTPS, websites can provide a secure environment for users, signified by a padlock icon in the browser’s address bar, reassuring users that their interactions with the site are protected against eavesdropping and tampering.

The other protocols listed serve different purposes: FTP is designed for transferring files, HTTP facilitates standard web page requests without encryption, and SMTP is used for sending emails. These do not inherently provide the same level of security that HTTPS does. Hence, the focus on secure data transmission directly correlates with the use of HTTPS as the correct answer.