What is the primary purpose of the APU?

The primary purpose of the Auxiliary Power Unit (APU) is to provide electric power. The APU is essentially a small turbine engine situated typically in the tail of the aircraft. Its main function is to generate electrical power when the main engines are not running or when there's insufficient power from the engines during ground operations. This electric power is essential for starting the main engines, powering the aircraft's electrical systems, and providing environmental control like air conditioning and cabin pressurization while on the ground.

While options such as fuel efficiency, hydraulic power, and flight control represent important aspects of aircraft operation, they are not the primary roles of the APU. The APU does contribute to hydraulic power indirectly, as some APUs can drive hydraulic systems. However, its principal designation remains electrical generation, making electric power the correct and most relevant answer to this question.