Which protocol is used primarily to transfer files between hosts?

The protocol primarily used to transfer files between hosts is the File Transfer Protocol (FTP). FTP is specifically designed for the purpose of transferring files over a network. It operates under a client-server model where a client connects to an FTP server to upload or download files.

FTP allows for various functionalities such as authentication, which is crucial for securing the file transfer process. It is widely used in scenarios ranging from web development, where files need to be uploaded to servers, to sharing large files over the internet.

In contrast, other protocols listed serve different purposes. For instance, Simple Mail Transfer Protocol (SMTP) is intended for sending emails, not for file transfer. Transfer Control Protocol (TCP) is a foundational communication protocol that ensures reliable transmission of data streams between computers, but it is not explicitly designed for file transfer. Lastly, File Control Protocol (FCP) is not a standard protocol used widely for file transfers and does not have the same recognition or functionality as FTP.

Therefore, FTP stands out as the correct choice due to its dedicated design for file transfers, making it a fundamental protocol in networking for this specific task.