When will the DC fuel pump automatically engage if there is no AC power?

The correct choice is that the DC fuel pump will automatically engage for APU operation only when there is no AC power. This is because the APU (Auxiliary Power Unit) requires a reliable source of fuel to operate, especially during ground operations when the aircraft's main engines are shut down.

In the absence of AC power, the DC fuel pump serves a crucial role by ensuring that the APU receives the necessary fuel supply. This is designed to facilitate the starting of the APU and to ensure it can function properly while providing electrical power and bleed air for systems that might be needed on the ground, thus maintaining operational capability without relying on the main engines or external power sources.

The other scenarios mentioned, such as engine shutdown, ready ground tests, or normal flight operations, do not trigger the automatic engagement of the DC fuel pump in the same way as APU operation does. During engine shutdown, for example, the aircraft typically relies on other systems to manage fuel flow, while ready ground tests may not necessitate the operation of the APU or its associated fuel systems. In normal flight operations, the AC fuel pumps are generally utilized, which makes the engagement of the DC pump for APU operation the correct context for this question.