What layer of the TCP/IP model does POP3 belong?

POP3, or Post Office Protocol version 3, operates at the Application Layer of the TCP/IP model. This layer is responsible for enabling user interfaces and providing services directly to end-user applications. POP3 is a protocol used by email clients to retrieve emails from a mail server, facilitating the downloading of email messages so that users can read them locally on their devices.

The Application Layer encompasses protocols that deal with high-level functions such as email (like POP3 and SMTP), file transfer (like FTP), and web browsing (HTTP and HTTPS). Since POP3 directly serves the purpose of accessing email, it is categorized within this layer.

In contrast, other layers such as the Transport Layer, which includes protocols like TCP and UDP, manage end-to-end communication and data transfer, rather than specific user applications. The Network Layer is concerned with the delivery of packets from one host to another across multiple networks, whereas the Internet Layer encompasses protocols that handle routing and addressing, like IP. All these layers work together, but POP3 is specifically an application-level protocol, aligning with the correct answer.