What are the three main sources from which an aircraft gets its supply?

The three main sources from which an aircraft gets its supply are indeed the engines, the Auxiliary Power Unit (APU), and a ground source. The engines are the primary source for thrust in flight, relying on fuel to operate and provide the necessary power for takeoff, climb, cruise, and descent. The APU serves as a secondary source that provides electrical power and pneumatic pressure while the aircraft is on the ground, allowing for system operations and environmental control without needing to engage the main engines. Additionally, ground sources include external power units that can provide electrical power when the aircraft is parked, enhancing efficiency and reducing fuel consumption while on the ground.

Other options, while referencing components that may supply certain needs, do not represent the main and standard sources of supply. For instance, solar power is not a typical source for aviation needs, and ground air conditioning pertains more to environmental control rather than comprehensive aircraft supply.