Which protocol retrieves email from a server and operates on port 110?

The correct choice is Post Office Protocol 3 (POP3) because it is specifically designed to retrieve email from a mail server. POP3 operates on port 110, allowing email clients to download emails from the server for offline access. This means that once the emails are downloaded, they are typically removed from the server, which distinguishes the protocol in how it manages email storage and access.

In contrast, Internet Message Access Protocol (IMAP) is another protocol used for retrieving email, but it typically operates on port 143 and allows users to view and manage their emails directly on the mail server, offering more functionalities for synchronization across multiple devices.

Simple Mail Transfer Protocol (SMTP) is used for sending emails, not retrieving them, and operates on port 25 or port 587, depending on the configuration.

Domain Name System (DNS) serves an entirely different function related to resolving domain names to IP addresses and does not deal with email retrieval. Its purposes and functionalities are distinct from any email-related protocols.