What color light indicates a runway or taxiway that is not in use?

Answer: C — Green

The color green is used to indicate a runway or taxiway that is not currently in use. This is part of a standardized system for airfield lighting, where green reflects that an area is clear and open for aircraft to utilize. The presence of green lights signals to pilots and ground personnel that they can safely access the area without the concern of active operations. This color coding is crucial for maintaining safety and situational awareness on the airfield, as it allows users to quickly assess the status of various airfield elements. Understanding these color codes helps enhance communication and reduce the risk of accidents on the ground.