Which of the following defines the communication link between a Web server and Web applications?

CGI, or Common Gateway Interface, is the standardized specification that defines the communication interface between a web server and web applications or scripts running on the server. It specifies how the server passes request data to an application and how the application returns output to the server, enabling dynamic content generation.

PGP (Pretty Good Privacy) is a data encryption and decryption program used for securing email and files, not for defining web server-to-application communication.

A firewall is a network security system that monitors and controls network traffic based on security rules, not a communication interface standard for web applications.

IETF (Internet Engineering Task Force) is a standards development organization responsible for Internet protocols, not a specific communication interface between web servers and applications.