Final answer:
The greatest demand is placed on an APU during ground operations and engine start. During takeoff, in-flight cruising, and landing, the main engines of the aircraft provide the necessary power.
Step-by-step explanation:
The greatest demand is placed on an APU (Auxiliary Power Unit) during ground operations and engine start.
During ground operations, the APU powers the aircraft's electrical systems and provides the necessary hydraulic power for various operations. Engine start also requires a significant amount of power from the APU to initiate the ignition process.
During takeoff, in-flight cruising, and landing, the main engines of the aircraft provide the necessary power, while the APU is typically in standby mode.