How does the APU contribute to onboard systems when on the ground?

The APU, or Auxiliary Power Unit, plays a crucial role in providing support to onboard systems while the aircraft is on the ground. Its primary functions include supplying electrical power and delivering bleed air, which are essential for various systems and operations during ground operations.

When the aircraft is on the ground, the main engines are typically not running, and the APU ensures that the aircraft can still operate necessary systems, such as lights, avionics, and environmental control systems. The provision of bleed air from the APU is particularly important for starting the main engines and for heating or cooling the cabin where required, allowing for a comfortable environment for passengers and crew before departure.

In contrast, the other options do not accurately represent the functions of the APU. The APU does not amplify engine performance, discharge waste to enhance cabin air quality significantly, or regulate cabin temperature massively by itself; rather, it assists in maintaining the necessary systems that support those functions when the engines are not in operation. Thus, the correct answer emphasizes the essential role of the APU in providing electrical power and bleed air to onboard systems while the aircraft is on the ground.