Which testing approach verifies a subsystem's behavior in isolation?

• A. Integration testing or component testing.
• B. System testing validating the entire product.
• C. Unit testing or component testing.
• D. Acceptance testing with customers.

Answer: (C)

Testing a single unit of code in isolation from the rest of the system focuses on validating its behavior under controlled conditions. This is unit testing; it targets one function or class (a small piece of the subsystem) and checks that its outputs match expectations for a variety of inputs, including edge cases. To isolate the unit, dependencies are replaced with mocks or stubs so nothing outside the unit affects the results. Because it concentrates on a narrow scope, unit testing is fast, repeatable, and helps you pinpoint where problems originate. Component testing is related in that it tests a larger component, but when the emphasis is on a subsystem behaving correctly on its own, unit testing is the most direct fit. By comparison, integration testing looks at how multiple units work together, system testing examines the whole product, and acceptance testing involves customers verifying the product.