Motorcycles and mopeds have a right to use...?

Answer: B — The whole of a lane

Motorcycles and mopeds are considered motor vehicles and therefore have the right to use the entire lane, just like other cars. Option A is incorrect because it implies that motorcycles and mopeds are only allowed to use the right side of the lane, which is not the case. Option C is incorrect because it limits their usage to only bicycle lanes, which is not safe for motor vehicles to use. Option D is also incorrect because it goes against traffic laws and safety regulations for motorcycles and mopeds to use the left side of the lane. Therefore, the correct answer is B, as motorcycles and mopeds have the same rights to use the full width of a lane as other motor vehicles.