Which of the following protocols is used primarily for sending emails?

The protocol primarily used for sending emails is Simple Mail Transfer Protocol (SMTP). This protocol is specifically designed to facilitate the sending of email messages from a client to a server or between servers. SMTP operates over the TCP/IP protocol suite and is the standard for email transmission across the internet.

Other protocols mentioned serve different purposes related to email. For instance, Internet Message Access Protocol (IMAP) is used for retrieving and managing emails from a mail server, allowing users to view their emails without downloading them, which is helpful for working across multiple devices. Post Office Protocol (POP) also aids in retrieving emails from a server, but it typically downloads and removes emails from the server, making them accessible only from the device that initiated the download. Network File System (NFS), on the other hand, is unrelated to email and is used for file sharing over a network.

Thus, within the context of sending emails specifically, SMTP is the designated protocol, making it the correct answer for this question.