The lockup pressure test verifies the pressure at which?

The lockup pressure test is designed to determine the pressure at which the regulator maintains its setting and effectively "locks up" when no propane is flowing through the system. This test is crucial because it helps ensure that the regulator can hold pressure without any gas being drawn off. Essentially, the lockup pressure indicates the stability and reliability of the regulator, confirming that it can effectively manage and control the pressure in the line when the demand for propane ceases. This maintains safety and efficiency in gas delivery systems.

The other choices do not accurately describe the purpose of the lockup pressure test. The concept does not relate to the tank being completely filled, the system reaching maximum capacity, or the functioning of the pressure relief valve. Each of those scenarios pertains to different aspects of propane system management and safety, but they do not reflect what is directly assessed through the lockup pressure test.