What would be a common use of the auxiliary power unit (APU)?

The auxiliary power unit (APU) serves several essential functions, with one of its primary roles being to provide power to the aircraft's electrical systems when the main engines are not running. During pre-flight checks, the APU is often employed to power avionics and other essential systems, allowing the crew to conduct necessary checks without relying on ground power units. This capability enhances efficiency and ensures that critical systems are operational before departure.

The other options, while relating to aviation operations, do not accurately describe the primary function of the APU. For instance, thrust during climb is generated by the aircraft's main engines, not the APU. Assisting during emergency landings typically involves the main engines and flight control systems rather than the APU. Additionally, while the APU can contribute to cabin air conditioning, its primary purpose on cruise flights does not focus on cooling; rather, it serves other onboard systems.