Which statement about test types and test levels is CORRECT?

The statement that functional, non-functional, and white-box testing can be performed at any test level is correct because it accurately reflects the flexibility in the application of different testing types across various stages of the software development lifecycle.

Functional testing primarily assesses whether the software meets specified requirements and can be applied at any level – from unit tests to system and acceptance tests. Non-functional testing, which evaluates aspects such as performance, usability, and reliability, also transcends various test levels and can be integrated both early and late in the development process based on the testing strategies employed.

White-box testing, focused on the internal structure of the application, can generally be applied at all levels as well, though practical application might be more common at the component and integration levels where the specific code and logic are accessible for examination. However, it is not strictly limited to just these levels, making it feasible to incorporate it at later testing stages too, as long as the necessary information is available.

This understanding reinforces the fact that various testing types are not confined to rigid boundaries defined by test levels, thereby supporting a comprehensive testing strategy tailored to the software under test.