The correct selection is B. Apex tests cannot make uncontrolled external web-service callouts. If Test.setMock() is omitted and the code attempts the callout, the test fails with the platform restriction rather than silently skipping the call or contacting the real endpoint. From a testing perspective, the goal is deterministic evidence that the solution behaves correctly under the conditions that matter. Tests should isolate external dependencies, use controlled data, exercise positive and negative paths, and reproduce realistic volume or user behavior where required without making production the first place defects are discovered. The alternatives do not provide equivalent evidence: they either validate only a narrow implementation detail or depend on conditions outside the test ' s control. For this scenario, the decisive point is therefore the platform behavior represented by the selected option (B. The test class fails and the developer will see a message stating: Methods defined as TestMethod do not support Web service callouts.). Applying that rule consistently keeps the implementation aligned with Salesforce-supported lifecycle practices and makes the resulting change easier to review, validate, and operate across environments.

Study Guide reference: Testing — Apex testing; test-data isolation; mocks; regression testing; performance/load testing; sandbox test strategy.