Step 1: Understanding HTTP.
HTTP, or HyperText Transfer Protocol, is the fundamental protocol for data communication on the World Wide Web.
Step 2: Role of HTTP.
It governs message formatting and transmission, dictating how web servers and browsers interact. For instance, when a user inputs a URL, the browser sends an HTTP request, and the server replies with the requested web page.
Step 3: Elimination of incorrect options.
- Option 1 is incorrect; email uses SMTP, IMAP, or POP3, not HTTP.
- Option 2 is incorrect; FTP is the file transfer protocol, not HTTP.
- Option 4 is incorrect; secure transmission involves HTTPS, which is HTTP with SSL/TLS encryption.
Step 4: Conclusion.
Consequently, Option 3 accurately defines HTTP as a set of rules for retrieving linked web pages.
Final Answer: \[\boxed{\text{HTTP is used to transfer and retrieve hypertext-linked web resources.}}\]