_______ is a protocol used by e-mail clients to download e-mails to your…

Concept. Electronic mail is not handled by a single protocol but by two distinct application-layer roles. A push (transfer) protocol carries a message outward — from the sending client to its mail server and onward between mail servers. A pull (access/retrieval) protocol works in the opposite direction: a mail client contacts the mailbox held on a mail server and transfers the stored messages down to the user’s own machine. Protocols that sit below the application layer, in the transport layer, define no mail operation at all; they only supply the reliable byte stream that the mail protocols run over.

Application. The blank describes a client-side program contacting a server-held mailbox and bringing the stored messages down to the local computer — the pull/retrieval role. Among the four choices, the Post Office Protocol (POP, current version POP3, TCP port 110) is the protocol defined for exactly that job: the client authenticates to the mailbox, lists the waiting messages, and transfers them onto the local machine. Retrieval by itself removes nothing from the server; a message is deleted only if the client marks it with the DELE command and the session then ends with a successful QUIT, which enters the UPDATE state where those marks are committed. Whether a copy remains on the server is therefore a matter of how the mail client is configured, not of the retrieval itself.

Cross-check — what each of the other choices is for:

• TCP — a transport-layer protocol. It provides the reliable, ordered, connection-oriented byte stream that application protocols (including every mail protocol here) are carried over, but it specifies no mailbox or message operation of its own.

• FTP — an application-layer protocol for transferring and managing arbitrary files between a client and a file server (TCP ports 20/21). It moves files, not mailbox messages.

• SMTP — the application-layer protocol for the push direction: submitting outgoing mail from the client to its server and relaying it server-to-server (TCP port 25, submission on 587). It delivers mail toward a mailbox, not out of one.

• POP — the application-layer protocol for the pull direction: retrieving stored messages from the mailbox onto the client machine.

Layer-and-role summary:

A useful contrast for the same pull role is IMAP, which also retrieves mail but is designed to keep messages stored and synchronised on the server across several clients rather than transferring them to one machine; IMAP is not among the choices here. The answer is therefore POP.