Which testing type validates an individual component in isolation?

• A. Unit testing.
• B. Integration testing.
• C. End-to-end testing.
• D. System testing.

Answer: (A)

Testing a single component in isolation means checking one small piece of code—like a function or a class—by itself, without the rest of the system affecting the test. You focus on its logic, inputs, outputs, and error handling. To keep it isolated, you replace dependencies with mocks or stubs so the test only exercises that unit. This setup gives fast, reliable feedback and makes it easy to pinpoint where a problem lives. In contrast, integration testing looks at how multiple units work together, ensuring interfaces and interactions are correct; end-to-end testing simulates real user scenarios across the full stack; system testing validates the whole application against requirements. So the thing that validates an individual component in isolation is unit testing.