What is an APU and its primary function?

The correct answer identifies an APU as an Auxiliary Power Unit, which plays a crucial role in aviation, particularly for aircraft on the ground. The primary function of the APU is to provide electrical power and air conditioning support while the main engines are not running. This capability is vital, as it allows aircraft systems, including lighting and air conditioning, to operate independently of the main engines during pre-flight preparations, boarding, and other ground operations.

The importance of the APU cannot be overstated, as it enhances the comfort of passengers and crew in maintaining an optimal cabin environment, as well as ensuring that critical systems are operational without relying on external power sources.

While the other options mention possible functionalities related to aircraft systems, they do not accurately define the APU and its primary purpose, which is firmly grounded in providing auxiliary power and air conditioning.