According to the syllabus, use case testing is a technique that tests the functionality of a system using scenarios based on use cases. A use case is a description of how an actor interacts with a system to achieve a goal. A use case defines the expected behaviour and possible variations of the system under test. The answer A is correct because test cases are designed to exercise the defined behaviours of the use case and its variations.
The other answers are incorrect because they are not true statements about use case testing. There can be multiple test cases for each use case, depending on the complexity and variations of the use case. Test cases can be derived from other sources besides activity diagrams, such as textual descriptions or flow charts. It is possible to measure use case test coverage by comparing the number of executed test cases with the number of defined test cases for each use case.
References: Certified Tester Foundation Level Syllabus, Section 4.5.1, page 49-50.