What are the different levels of testing (unit, integration, system) and their objectives?

• A. Unit tests verify individual components; integration tests check interactions between components; system tests validate the entire system against requirements.
• B. Unit tests validate requirements; integration tests verify user acceptance; system tests check performance.
• C. Unit tests test code style; integration tests test compilation; system tests test deployment scripts.
• D. Unit tests verify individual components; integration tests check interactions between components; system tests validate the entire system against requirements.

Answer: (D)

Testing at different levels targets different scopes, from tiny pieces of code to the whole product. Unit tests verify that individual components, such as a function or a class, behave correctly in isolation. They focus on the unit’s deterministic behavior and are designed to run quickly, so you can run many of them during development.

Integration tests check how those pieces work together. They validate the interfaces between components, the data exchange, and that combined parts interact as expected, catching issues that only appear when modules are wired together.

System tests look at the entire system against the stated requirements. They simulate real-world usage and verify end-to-end behavior, along with non-functional aspects like reliability and performance, in an environment that resembles production.

This combination gives a solid testing approach: unit tests catch small defects early, integration tests catch interaction issues, and system tests ensure the product as a whole meets user needs.

Other options mix up the objectives—for example assigning requirements validation or performance to unit tests, or focusing on style, compilation, or deployment scripts rather than the intended levels.