Apex-managed sharing gives the development team substantial control over who receives record access, so it must be tested from the perspective of different users. System.runAs() is the appropriate test technique among the choices because it allows the test to execute sharing-sensitive behavior under representative user contexts and verify both positive and negative access outcomes. The test suite should include users who are supposed to receive access and users who must remain restricted, then confirm that the managed share entries produce exactly the intended visibility. A keyword such as without sharing would weaken record enforcement rather than validate it, and with sharing does not enforce Field-Level Security. In production code, record sharing, object CRUD, and FLS still have to be handled as separate concerns; runAs() simply provides a reliable way to exercise user-context behavior in tests. Study Guide reference: Access to Records - Apex-managed sharing, System.runAs(), Private OWD, user-context testing, and validation of least-privilege access.