After takeoff, what causes the bleeds to transfer from APU to engines?

The correct answer indicates that when the thrust lever is retarded from TOGA (Takeoff Go-Around) mode with the landing gear up and the flaps set to 20 degrees or less, the bleed air transfer from the APU to the engines occurs. This transition is part of the aircraft’s normal operation after takeoff when it has achieved an appropriate configuration for climb and normal flight.

In this scenario, the aircraft’s systems are designed to automatically switch the source of bleed air from the APU to the engines once it is safe and optimal to do so. As the aircraft increases performance during the climb phase, using bleed air from the engines ensures that sufficient power and efficiency are maintained, rather than relying on the APU, which is primarily used during ground operations.

The other choices do not accurately describe the operational logic behind the bleed air transfer. Switching to engine bleed mode may occur as part of the process but does not independently trigger the transfer. Losing power to the APU would not cause a planned and safe transfer of bleed air; in fact, it would complicate the situation. Activating the engine start sequence is unrelated to this specific transfer, as it pertains to engine ignition rather than bleed air management during normal flight operations.