According to the syllabus, non-functional testing is a type of software testing that verifies non-functional aspects or attributes of a software product or system, such as performance, usability, reliability, security, etc. Non-functional testing evaluates how well a system operates or behaves under certain conditions or scenarios, rather than what functions or features it provides. The answer D is correct because it best exemplifies non-functional testing. Tests which capture the time it takes to save a file are examples of performance testing, which is a type of non-functional testing that measures how fast or responsive a system is under various workloads or situations. The other answers are incorrect because they exemplify functional testing or structural testing, which are not types of non-functional testing. Tests based on the internal structure of a component or system (A) are examples of structural testing, which is a type of software testing that verifies the implementation or code of a software product or system. Tests related to ‘what’ the system should do (B) are examples of functional testing, which is a type of software testing that verifies the functionality or behavior of a software product or system. Tests which calculate overtime pay for those employees entitled to such © are also examples of functional testing, as they verify a specific function or feature of a software product or system.

References: Certified Tester Foundation Level Syllabus, Section 1.2.1, page 10-11.