Where may functional testing be performed?

Functional testing is a test type focused on evaluating what a system does, based on its functional requirements and specifications. This test type is not confined to a specific test level. It can be applied at every stage of the testing process. For instance, at the component level, it verifies the functionality of individual units. At the integration level, it checks the functional behavior of interacting components. At the system and acceptance levels, it validates that the complete system functions according to end-to-end business requirements and user expectations. Therefore, functional testing is applicable at all test levels.

A. This is incorrect because it omits component and integration testing, where verifying the functionality of individual parts and their interactions is crucial.
C. This is incorrect as it wrongly excludes component and integration testing, which are fundamental stages for performing functional tests on modules and their interfaces.
D. This is incorrect because it severely restricts functional testing to a single level, ignoring its critical role in earlier stages like component, integration, and system testing.

1. ISTQB® Certified Tester Foundation Level Syllabus (Version 2018):

Section 2.2.1, "Functional Testing," states: "Functional tests can be performed at all test levels (e.g., testing the functionality of an individual component during component testing, or testing the functionality of the end-to-end business process during system testing)." This directly supports that functional testing is applicable at all levels.

2. Black, R. (2019). ISTQB Certified Tester Foundation Level Syllabus 2018. Rocky Nook.

Chapter 2, "Testing Throughout the Software Development Lifecycle," explains the relationship between test levels and test types. It clarifies that test types like functional testing are orthogonal to test levels and can be executed at any level. For example, on page 68, it discusses how functional testing applies to component testing, integration testing, system testing, and acceptance testing.

3. Jorgensen, P. C. (2013). Software Testing: A Craftsman's Approach, Fourth Edition. CRC Press.

Chapter 1, "A Math and Logic Foundation," Section 1.3, "The V-Model," discusses the relationship between development phases and test levels. The text implicitly and explicitly supports that functional verification (a form of functional testing) occurs at every level, from unit to system testing, to check against the requirements defined at the corresponding development stage. (See Figure 1.6, page 18).