Which of the following is essential for the proper functioning of an APU?

The proper functioning of an Auxiliary Power Unit (APU) is highly dependent on a clean and efficient fuel supply. An APU is designed to provide power to auxiliary systems on the aircraft, especially when the main engines are not running. Its operation requires a reliable source of fuel that ensures optimal combustion, stability, and efficiency. If the fuel supply is contaminated or not functioning efficiently, it can lead to operational issues, reduced performance, and potentially costly mechanical failures.

On the other hand, while an external fuel source can be beneficial, it is not essential since the APU typically uses the aircraft's onboard fuel supply. Visible maintenance checks are important for the APU’s upkeep, but they do not directly contribute to its functioning. Lastly, while minimizing environmental impact is a commendable goal, it is not a critical factor for the APU's operation. In summary, ensuring a clean and efficient fuel supply is vital for the APU to function optimally and to support the aircraft's operational capabilities.