What does "D" signify on an airport diagram?

On an airport diagram, "D" signifies a deicing area. Deicing is a critical process, especially in cold weather, wherein an aircraft is treated to remove or prevent ice accumulation on its wings and other surfaces. This ensures safe takeoff and flight operations. The deicing area is typically marked on airport diagrams so that pilots and ground personnel can easily identify it and navigate safely during winter operations.

The other options, while related to airport operations, do not accurately represent what "D" signifies. Understanding these designations is essential for pilots and air traffic control personnel to ensure efficient and safe ground handling of aircraft, particularly under specific weather conditions.