What is the appropriate action when the APU is in operation and the AC pump is unavailable?

When the Auxiliary Power Unit (APU) is in operation and the AC pump is unavailable, activating the DC fuel pump is the most appropriate action. The DC fuel pump provides a reliable alternative for fuel pressure and flow when the AC pump, typically used for main fuel supply, is not functional. This ensures that the engines are supplied with fuel in a safe and efficient manner, maintaining operational integrity.

Utilizing the DC fuel pump allows the aircraft to continue its operations without significant disruption, as it is designed to serve as a backup to the AC system. This option seamlessly supports the functionality of the fuel system during an APU operation, ensuring fuel delivery to the engines is maintained as needed.

Other options, such as fallback to manual override or using alternate fuel sources, may complicate the situation or might not provide the necessary immediate response to fuel delivery requirements. Turning off the APU could disrupt electrical power and other systems that depend on its operation, making activating the DC pump the most prudent choice.