Which protocol is associated with the Application Layer for email communication?

The protocol associated with the Application Layer for email communication is SMTP, which stands for Simple Mail Transfer Protocol. SMTP is specifically designed for sending and transferring email messages between servers. It operates at the Application Layer of the OSI model, defining the rules and standards for how emails are formatted and sent.

In the context of email communication, SMTP plays a crucial role, handling the sending process from the sender's email client to the recipient's mail server. It ensures the reliable transfer of messages by specifying the necessary commands and response codes.

The other protocols mentioned serve different purposes. FTP (File Transfer Protocol) is used for transferring files between a client and a server. HTTP (Hypertext Transfer Protocol) is used primarily for transferring web pages on the internet. Modbus is a protocol used for communication in industrial environments, particularly in automation systems. Each of these serves important functions in their respective domains but does not directly relate to the process of sending and handling email, which is why SMTP is the correct choice for email communication in the Application Layer.