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.