Step 1: Understanding HTTP.
HTTP stands for HyperText Transfer Protocol. It is the foundation of data communication for the World Wide Web.
Step 2: Role of HTTP.
It defines how messages are formatted and transmitted, and how web servers and browsers should respond to commands.
For example, when a user enters a URL, the browser sends an HTTP request to the web server, and the server responds with the requested web page.
Step 3: Elimination of incorrect options.
- Option 1 is incorrect because email communication uses SMTP, IMAP, or POP3, not HTTP.
- Option 2 is incorrect because a file transfer protocol is FTP, not HTTP.
- Option 4 is incorrect because secure transmission uses HTTPS, which is HTTP + SSL/TLS encryption.
Step 4: Conclusion.
Therefore, the correct definition is Option 3: HTTP is a set of rules used to retrieve linked web pages across the web.
Final Answer:
\[
\boxed{\text{HTTP is used to transfer and retrieve hypertext-linked web resources.}}
\]