At which test levels may functional tests be executed?

Functional tests are designed to verify that a software application behaves as expected according to its specifications. This type of testing can be performed at various levels of software development, which include unit testing, integration testing, system testing, and acceptance testing.

At the unit level, functional tests can be executed to confirm that individual components or functions operate correctly in isolation. At the integration level, these tests validate interactions between integrated components to ensure that they work together as intended. System testing involves evaluating the complete system to ensure that all functional requirements are met, making it a critical phase for functional testing. Finally, acceptance testing is performed to determine whether the software meets the business requirements and is ready for deployment, further emphasizing the importance of functional tests at this stage.

Since functional tests can indeed be executed at all these levels to validate expected behaviors and outcomes, it is accurate to state that they can be performed throughout the entire software development lifecycle.