What color should stop lights be when functioning?

Answer: D — Red

Stop lights are universally recognized as red when they are functioning properly. The red signal indicates that vehicles must come to a complete stop and must not proceed until the light changes to green. This color is critical for ensuring safety at intersections, as it helps to control the flow of traffic and prevent accidents. The use of red for stop signals is standardized in traffic regulations, making it an essential part of driving education. Understanding this signal is crucial for all drivers, as it plays a key role in traffic management and compliance with the law.