POP (Post Office Protocol) is used by email clients to download emails from the server to the user's computer.

Understanding Email Protocols
When it comes to downloading emails from a server to a computer, various protocols are employed. Among them, the most relevant for this purpose is POP, which stands for Post Office Protocol.

What is POP?
POP is designed specifically for retrieving emails from a mail server. Here’s a breakdown of how it functions:
- **Download and Store**: POP allows email clients to download emails from the server to the local device. Once downloaded, emails are typically removed from the server, making them accessible offline.
- **User Control**: Users have the option to configure their email client to either delete emails from the server after downloading or keep a copy on the server for future access.

Comparison with Other Protocols
- **SMTP (Simple Mail Transfer Protocol)**: SMTP is used for sending emails rather than downloading. It works as the outgoing mail server, handling the transmission of emails sent from client to server.
- **FTP (File Transfer Protocol)**: FTP is not used in email contexts; it’s designed for transferring files between computers over a network.
- **TCP (Transmission Control Protocol)**: While TCP is a foundational communication protocol that supports various applications, including email, it is not specifically dedicated to email retrieval.

Why Choose POP?
- **Offline Access**: One of the primary advantages of POP is the ability to access emails without an internet connection once downloaded.
- **Simplicity**: POP is straightforward and effective for users who primarily access their emails from a single device.
In summary, POP is the correct answer for downloading emails to a computer, as it is specifically designed for this purpose, unlike SMTP, FTP, or TCP.