Which protocol should Nathan use to send large, high-resolution graphic files to his co-workers?

Using File Transfer Protocol (FTP) is the most effective choice for sending large, high-resolution graphic files to co-workers. FTP is specifically designed for transferring files over the internet, supporting large file sizes and enabling efficient uploads and downloads. It allows users to connect to a server and transfer files directly, making it ideal for sharing large datasets or graphic files without the size limitations often encountered with other methods.

FTP operates by establishing a connection that allows for the seamless transfer of files, and it can handle various types of files, including those that are large in size and require significant bandwidth. This is especially important when dealing with high-resolution graphics which can be quite large.

In contrast, using email generally imposes strict attachment size limits that can hinder the transfer of large files. HTTP is primarily used for accessing and managing resources on the web and is not specifically optimized for file transfers in the way FTP is. IMAP is a protocol for retrieving emails and managing them on a server, not suitable for transferring large files at all. Thus, FTP stands out as the best option for Nathan's needs in this scenario.