Which of the following protocols would likely be used for email transmission?

The correct choice is SMTP, which stands for Simple Mail Transfer Protocol. This protocol is specifically designed for sending emails across the internet. SMTP functions as a set of communication rules that dictate how email messages are transmitted between mail servers. When a user sends an email, their email client uses SMTP to relay the message to the recipient's mail server, ensuring that the message is formatted properly for transmission.

For context, HTTP, which stands for HyperText Transfer Protocol, is primarily used for transferring web pages and files on the internet rather than email. IPSec is a suite of protocols used for securing internet protocol (IP) communications by authenticating and encrypting each IP packet in a communication session, which is not directly related to email transmission. Ethernet, on the other hand, is a network technology used for local area networks (LAN); it operates at the data link layer and governs how data packets are formatted and transmitted over a physical medium, serving as the underlying transport rather than an application for email functionality.

Therefore, SMTP is the most appropriate protocol for email transmission due to its specific design and purpose.