Which of the following describes a test case?

A test case is primarily defined as a set of conditions or variables under which a tester assesses whether a system or software application is functioning as intended. It includes specific inputs, execution conditions, and expected results that allow testers to verify if the software behaves correctly. The structured approach of a test case enables testers to systematically check various aspects of application functionality, and interoperability, or to verify that changes to the code behave as expected.

The other choices describe different testing concepts. For instance, a document that describes the objectives and expected outcomes of the testing process refers more to a test plan or test strategy rather than a specific test case. A listing of defects found during testing pertains to defect management and is typically documented in bug tracking systems, not in individual test cases. Lastly, a measurement of the software's performance under specific conditions aligns more with performance testing metrics rather than the structure of a test case itself. Therefore, the correct answer highlights the comprehensive nature of test cases as essential tools in the software testing process.