A written test is said to be comprehensive when it...

A written test is considered comprehensive when it samples liberally whatever is being measured. This means that the test covers a wide array of content and assesses various aspects of the subject matter, providing a broad overview of the knowledge or skills expected from the test takers.

A comprehensive test isn't just limited to testing knowledge from one section or topic but encompasses multiple facets of the curriculum, ensuring that all relevant content areas are represented. This is essential for gauging overall competency and understanding in a subject, rather than just a narrow slice.

The other options, while relevant to testing, do not capture the essence of comprehensiveness as effectively. For instance, including all levels of difficulty alone does not ensure a broad sampling of the subject; it focuses more on the challenge level of questions. Similarly, providing a wealth of background knowledge might contribute to preparation but does not reflect the testing of knowledge itself. Testing understanding across various topics is close but does not emphasize the breadth of sampling as thoroughly as the correct answer does. In summary, comprehensive testing is about achieving a well-rounded assessment that effectively measures the full spectrum of what is being tested.