The correct answer is D. SMTP (Simple Mail Transfer Protocol).

According to CompTIA A+ Core 1 220-1201 objectives under Networking:

SMTP is the standard protocol used to send emails from a client to a mail server or between servers.

POP3 (Post Office Protocol 3) and IMAP (Internet Message Access Protocol) are used for receiving emails, not sending them.

LDAP (Lightweight Directory Access Protocol) is used for accessing and managing directory services, such as authentication and organizational information, not for email transport.