Which one of the following protocols allows email clients to download their messages from an email server?

The protocol that allows email clients to download their messages from an email server is Post Office Protocol version 3 (POP3). POP3 is specifically designed to enable users to retrieve emails from their mail servers and store them locally on their devices. When using POP3, emails are typically downloaded and stored locally, allowing users to access their messages without the need for a continuous connection to the internet.

POP3 works by connecting to the mail server and downloading messages to the email client, often removing the emails from the server in the process unless configured otherwise. This simplicity makes it effective for users who prefer to manage their emails offline.

In contrast, Internet Message Access Protocol version 4 (IMAP4) also allows email retrieval but is designed for users who want to manage their emails directly on the server. IMAP4 enables multiple devices to access the same mailbox, keeping messages synchronized across devices and allowing for better management of folders and message states.

Simple Mail Transfer Protocol (SMTP) is primarily used for sending emails rather than retrieving them. It works alongside POP3 or IMAP4 rather than functioning as a retrieval protocol. Hypertext Transfer Protocol (HTTP) is unrelated as it is used for transferring web pages rather than managing email.