Which UML diagram is primarily used to represent functional requirements?

A Use Case Diagram is primarily used to represent functional requirements because it visually depicts the interactions between users (actors) and the system to achieve specific goals. This diagram outlines the various use cases—essentially the functionalities or services that the system provides from the user's perspective. By illustrating these interactions, a Use Case Diagram helps stakeholders understand what the system is supposed to do, outlining the requirements functionality in a clear and accessible manner.

In contrast, a Class Diagram focuses on the structure of the system, defining the classes, their attributes, and the relationships between them rather than functional requirements. A Sequence Diagram illustrates how objects interact over time, primarily detailing the order of message exchanges but not expressing what needs to be done in terms of functionality. An Activity Diagram represents workflows or processes, showing the flow of control or data but is more about how processes are executed rather than the specific functional requirements of the system.