Under what condition should you use full beam lights?

Answer: B — When the road ahead is clear of traffic

Full beam lights should only be used when the road ahead is clear of traffic, as these lights can blind other drivers. In heavily populated areas, it is important to switch to low beam lights to avoid blinding other drivers and causing potential accidents. Using full beam lights at all times, as suggested in option C, is incorrect and can be dangerous for other drivers on the road. Option D, never using full beam lights, is also incorrect as they can be useful when the road ahead is clear of traffic, providing better visibility.