Where does the inlet air for the APU enter the aircraft?

The correct entry point for the inlet air for the Auxiliary Power Unit (APU) on the A320 is through the automatic flap on the lower side of the fuselage. This design allows for efficient air intake necessary for the APU's operation. The APU requires a sufficient airflow to ensure proper combustion for providing bleed air and electrical power when the aircraft is not in flight, and the placement of the inlet on the lower side is strategically designed to optimize performance while also minimizing the risk of foreign object debris entering the unit.

In contrast, other options do not accurately reflect the actual configuration of the A320. For instance, the upper fuselage and engine cowling do not serve as intake points for the APU, and although the wing leading edge might be involved in different aerodynamic functions, it is not related to the APU air intake. Thus, the automatic flap's role is crucial in allowing the APU to operate effectively while ensuring that air enters from a location that enhances both safety and performance.