Which of the following protocols is commonly used for sending secure email?

The correct choice is based on the role of the Simple Mail Transfer Protocol (SMTP) in the context of email communication. SMTP is specifically designed for sending emails across networks, and it plays a pivotal role in the email transmission process. When secure email transmission is required, SMTP can be extended to support encryption protocols such as TLS (Transport Layer Security). This allows emails to be securely transmitted, protecting the contents from potential interception or tampering during transit.

In contrast, the other protocols listed serve different purposes. HTTP (Hypertext Transfer Protocol) is primarily used for transferring web pages over the internet, not for email. DNS (Domain Name System) is used for resolving domain names into IP addresses, helping systems locate each other on the network, but it does not pertain to email sending or encryption. POP3 (Post Office Protocol version 3) is utilized for retrieving emails from a mail server, which is separate from the process of sending emails. Thus, SMTP is clearly established as the protocol associated with sending and securing email communications.