Answer :
Final answer:
An aircraft exiting a runway is considered clear when the nosewheel crosses the runway holding position markings.
Explanation:
An aircraft exiting a runway is considered clear of the runway when the nosewheel crosses the runway holding position markings. This is because the runway holding markings indicate the boundary between the runway and the taxiway, and crossing them ensures that the aircraft is no longer on the runway. The main landing gear clearing the runway threshold (option b) is not sufficient because the aircraft could still be partially on the runway. Coming to a complete stop at the exit taxiway (option c) is not necessary for an aircraft to be considered clear of the runway, and it would cause unnecessary delays in busy airport operations. The tower controller giving clearance to exit (option d) is not the defining factor for determining when an aircraft is clear of the runway, as the controller's clearance could be given before the aircraft physically exits the runway.
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