Which protocol is commonly used for transmitting email?

The correct choice for the protocol commonly used for transmitting email is Simple Mail Transfer Protocol (SMTP). SMTP is specifically designed for sending emails from one server to another, facilitating the transfer of messages across the Internet. This protocol operates on a push model, allowing email clients and servers to communicate effectively for the purpose of sending and relaying messages.

It's important to distinguish the roles of the other protocols: HTTP is primarily used for transferring web pages and other information on the Internet, FTP is intended for transferring files between computers, and IMAP is used for retrieving messages from a mail server, allowing users to read their emails but not to send them. Therefore, while IMAP plays a critical role in email management, it does not serve the purpose of sending emails, which is where SMTP excels. This specialization makes SMTP the go-to protocol for email transmission.