Which of the following is a function of an Auxiliary Power Unit (APU)?

An Auxiliary Power Unit (APU) is an important component of aircraft systems, serving specifically to supply power for onboard systems when the engines are not running. This might occur during ground operations when the main engines are shut down, yet there is a need for electrical power to operate various systems such as lighting, air conditioning, and other avionics.

The APU allows the aircraft to remain operational on the ground without needing to rely on external power sources, thus enhancing efficiency during boarding and pre-flight checks. This functionality is vital for maintaining a comfortable environment for passengers and crew while the aircraft is stationary.

Regarding the other options, providing emergency exits refers to safety features of the aircraft's design rather than something the APU does. Controlling cabin pressure is managed by the aircraft's pressurization systems, which operate differently and do not involve the APU. Lastly, assisting with flight navigation is a function of the aircraft's navigation systems and instrumentation, independent of the APU's purpose or functionality.