What layer contains protocols that perform higher-level functions such as email and file transfer?

The correct choice is the Application Layer, which is responsible for providing network services directly to end users. This layer contains protocols that define how applications communicate over the network. Prominent examples include protocols such as HTTP (Hypertext Transfer Protocol) for web browsing, SMTP (Simple Mail Transfer Protocol) for sending emails, and FTP (File Transfer Protocol) for transferring files.

At this layer, the focus is on the interaction between software applications and the network, allowing users to perform tasks like sending emails or transferring files. Unlike lower layers, which handle data transmission and routing, the Application Layer specifically addresses how applications communicate and what data is exchanged.

The Transport Layer primarily focuses on providing reliable data transport, managing end-to-end communication, and ensuring that packets are delivered error-free and in sequence, but it doesn't deal directly with higher-level functions like email or file transfer. The Internet Layer is responsible for addressing and routing packets across the network, while the Link/Network Interface Layer deals with the physical transmission of data. Each of these lower layers plays a crucial role in the overall networking model, but they do not encompass higher-level application functionalities directly.