Which of the following vehicles must have mud flaps when operated on public roads?

The requirement for mud flaps primarily addresses the need to reduce debris and water spray from larger vehicles, which can create hazardous driving conditions for other road users. Vehicles with four or more tires on the rear axle are more likely to kick up dirt, water, and other materials that could affect visibility and safety on the road. By mandating mud flaps on these vehicles, it reduces the risk of these materials being projected onto smaller vehicles and pedestrians.

Single truck tractors and pole trailers are exempt from this requirement because of their design and the nature of their operation, which typically generates less debris. Therefore, the correct answer encompasses a broad range of vehicles equipped to transport loads or provide services while maintaining safety standards on public roads.