Which vehicles are not allowed to use a road sign-posted like this?

This road sign means that motorcycles are prohibited from using the road where the sign is posted. The red circle with the motorcycle symbol inside indicates a restriction specifically for motorcycles, not for other vehicles like cars or trucks. According to German traffic rules, when you see this sign, all types of motorcycles, including mopeds and motorbikes, are not allowed to enter or drive on that road. Cars and trucks are not affected by this sign unless other signs specify otherwise. Your answer was correct because only motorcycles are banned in this situation. In the future, look for the vehicle symbol inside the red circle to know which vehicles are excluded.