Which of the following is NOT a function of the APU?

The correct answer is based on the specific functions of the Auxiliary Power Unit (APU) in an aircraft. The APU serves several essential purposes, primarily related to providing energy for systems while the main engines are not running.

One of the primary functions is to serve as a self-contained source of pneumatic power, which is critical for starting the main engines and providing air for environmental control systems. Additionally, the APU provides emergency electrical power when the main electrical system becomes inoperative, allowing essential systems to remain functional.

It's important to recognize that while fuel tanks are integral to the aircraft's operation, the APU does not have a direct role in the operation of fuel tanks. The management and operation of fuel tanks are primarily overseen by the aircraft's fuel system, which involves pumps, valves, and gauges managed independently from the APU. Hence, the operation of fuel tanks is not a function of the APU, making this the correct answer to the question.