Which statement best differentiates an integration test from a unit test?

• A. Integration tests verify a single function in isolation.
• B. Integration tests verify how combined parts work together, while unit tests verify a small part in isolation.
• C. Unit tests verify system performance.
• D. Unit tests require a GUI.

Answer: (B)

The main idea here is the scope of what each test type checks. A unit test focuses on a small piece of code—usually a single function or method—and verifies its logic in isolation, often by replacing any dependencies with mocks or stubs so you control the environment. An integration test, on the other hand, checks how multiple parts of the system work together, ensuring that interfaces and data flow between components are correct when they interact.

So, the best statement is that integration tests verify how combined parts work together, while unit tests verify a small part in isolation. The other options mix up the scope or purpose: testing a single function in isolation is what unit tests do, not integration; system performance isn’t the typical focus of unit tests; and unit tests don’t require a GUI.