What protocol allows users to send documents and images across networks?

The Hypertext Transfer Protocol (HTTP) is the correct choice because it is specifically designed for transmitting hypertext documents and multimedia content over the web. HTTP facilitates the transfer of various types of data, including text, images, videos, and other files, between a web server and a client, typically a web browser. This protocol underpins how data is exchanged and accessed on the internet through Uniform Resource Locators (URLs), making it fundamental for web browsing and the sharing of documents and images online.

In contrast, while File Transfer Protocol (FTP) also serves the purpose of transferring files, it is primarily used for the transfer of files between a client and a server without the context of web browsing. Simple Mail Transfer Protocol (SMTP) focuses specifically on sending and routing emails, which doesn't encompass the broader document and image transfer across networks. Transmission Control Protocol (TCP), on the other hand, is a transport layer protocol that is responsible for ensuring reliable transmission of data but does not itself transmit documents or images; it works in conjunction with other protocols like HTTP to facilitate the delivery of files over the internet.