What signal device must be used by vehicles on the movement area?

Answer: B — lighted whirling beacon or rotating light

The use of a lighted whirling beacon or rotating light on vehicles operating in the movement area of an airfield is crucial for safety and visibility. These devices are specifically designed to enhance the visibility of vehicles, particularly in the dynamic and potentially chaotic environment of an airfield, where aircraft and ground vehicles are often present. The bright, rotating light serves to alert pilots and ground personnel to the vehicle's presence, reducing the risk of collisions and mishaps. In contrast, while a reflective vest can increase the visibility of personnel, it does not apply to vehicles. Similarly, flags and road flares, although useful in certain scenarios, do not provide the same level of consistent visibility as a lighted beacon. Flags can be hard to see when conditions are poor or when vehicles are moving at speed, and road flares are primarily used for emergency situations and not as a standard safety measure for vehicles in transit on the airfield.