At controlled airports, when should an aircraft stop according to taxiway holding position markings?

The correct choice indicates that an aircraft should stop on the solid line side of the taxiway holding position markings unless cleared by Air Traffic Control (ATC). This procedure is crucial for maintaining safety at controlled airports where precise management of aircraft movements is necessary to prevent conflicts with other aircraft and ensure safe operations on taxiways and runways.

The solid line represents a clear boundary indicating where the pilot must stop when approaching a runway. By stopping on the solid line side, pilots ensure they do not inadvertently enter a runway that may be active, where other aircraft could be taking off or landing. Being cleared by ATC means that the pilot has received specific permission to cross the holding line and enter the runway environment, which is essential for coordinated and safe operations in controlled airspace.

Understanding this holds particular importance, as it reinforces the role of ATC in providing instructions that ensure the safety and efficiency of all operations within the airport environment.