Which of the following protocols uses the port 25?

(A) HTTPS
(B) HTTP
(C) SMTP
(D) SSH

SMTP is the correct answer.

Simple Mail Transfer Protocol (SMTP) – Port 25. SMTP is the protocol used for sending e-mail over the Internet. Your e-mail client (such as Outlook, or Mac OS X Mail) uses SMTP to send a message to the mail server, and the mail server uses SMTP to relay that message to the correct receiving mail server. Basically, SMTP is a set of commands that authenticate and direct the transfer of electronic mail.