Which of the following sentences is correct?

Functional and Structural tests are two different test design techniques, not test levels or test types. They can be applied at any test level (Unit, Integration, System, Acceptance) and for any test type (Functional, Non-functional, Regression, Maintenance). Therefore, option A is the correct answer. Option B is incorrect because Functional tests can be performed at any test level, not just Unit and Integration. Option C is incorrect because Functional and Structural are not test levels, but test design techniques. Option D is incorrect because Structural tests can be performed at any test level, not just Unit.

References: ISTQB Certified Tester Foundation Level Syllabus 2018 v3.1.1, Chapter 4.1.2 Test Design Techniques1