What is the function of the Auxiliary Power Unit (APU) on an aircraft?

The Auxiliary Power Unit (APU) primarily functions to supply ground power when the aircraft's engines are not operating. This capability is essential because it allows the aircraft to maintain electrical systems, lighting, and environmental control while on the ground, particularly when the engines are shut down. The APU acts as a small power generator that ensures that critical systems continue to function during ground operations, such as pre-flight checks and cabin conditioning before the main engines are started.

This option is crucial for operational efficiency and passenger comfort, making it an integral part of ground handling procedures at airports. The other functions listed, although important in the context of aircraft operations, either describe systems or components that are not the primary role of the APU. For instance, while an APU can provide some degree of emergency power, its primary purpose relates to ground power supply rather than acting as a backup power source in-flight.