What is primarily needed for the APU fuel pump to operate when engines are off?

The operation of the Auxiliary Power Unit (APU) fuel pump, particularly when the engines are off, requires electrical power to function effectively. Specifically, the main battery provides this necessary electrical power, allowing the fuel pump to draw fuel from the aircraft's fuel system and supply it to the APU.

While fuel availability from the main tank is crucial for the APU to run once it starts, it does not directly impact the operation of the fuel pump itself. The pump's function is to deliver fuel to the APU, and without electrical power from the main battery or another source, the pump would be unable to operate, regardless of fuel availability.

Similarly, the activation of a backup generator and a system readiness check are important components of the overall operational readiness of the electrical systems in an aircraft, but they do not play a direct role in the basic functioning of the APU fuel pump when the engines are off. Thus, the key requirement for the APU fuel pump's operation at that time is the electrical power supplied by the main battery.