What is the purpose of a use case diagram?

A use case diagram is a type of behavioral diagram defined by UML (Unified Modeling Language).

The primary purpose of a use case diagram is to model the dynamic behavior of a system from a user's perspective.

It shows interactions between actors (users or other systems) and the system itself, represented by use cases (specific functionalities or services).

Use case diagrams help stakeholders understand what the system will do and how users will interact with it.

Option B is incorrect because while use cases help gather requirements, defining system requirements is a broader process that involves more than just use case diagrams.

Option C is incorrect because describing the static structure is the role of class diagrams or object diagrams, not use case diagrams.

Option D is incorrect because internal workings of a system are better described by activity diagrams, sequence diagrams, or class diagrams.

Therefore, the correct answer is Option A: To model dynamic behavior of a system.