Which network protocol is used for sending email?

The answer is B, as SMTP, or Simple Mail Transfer Protocol, is the standard protocol used for sending emails across the Internet. It functions by facilitating the transfer of email messages from a client to a server or between servers, allowing for the communication needed to send messages. SMTP typically operates over TCP port 25, though other ports like 587 are also commonly used for secure transmissions.

In contrast, FTP (File Transfer Protocol) is designed for transferring files between computers but is not used specifically for email purposes. HTTP (Hypertext Transfer Protocol) is used primarily for transferring web pages and resources on the Internet, rather than handling email. IP (Internet Protocol) is a fundamental networking protocol that deals with addressing and routing data packets but does not encompass email transmission directly. Thus, SMTP is clearly the appropriate choice for sending email.