Which protocol is responsible for sending email messages and operates on port 25?

Simple Mail Transfer Protocol (SMTP) is the correct choice because it is specifically designed for sending email messages over the internet. SMTP operates on port 25, facilitating the transmission of emails from the sender's mail server to the recipient's mail server.

This protocol works by defining a set of commands and responses that govern how email messages are sent and received, making it essential for the delivery of emails across networks. Its role is primarily in the outgoing mail process, as opposed to protocols that handle incoming email such as Post Office Protocol (POP3) or Internet Message Access Protocol (IMAP).

Both DNS and DHCP have different purposes; DNS translates domain names into IP addresses, while DHCP is responsible for assigning IP addresses to devices on a network. Neither of these protocols is involved in the sending of email messages, which is why they are not the correct choice.