What requirement exists for mud flaps on trucks with four or more tires?

For trucks that have four or more tires, the requirement states that mud flaps must be installed. This regulation is primarily in place to reduce road splash and debris, which can be a hazard for other vehicles and pedestrians. By mandating the installation of mud flaps, trucks help ensure that water, mud, and other materials are not thrown onto the roads as much, thereby improving safety. Properly installed mud flaps also protect the truck's undercarriage and reduce wear and tear from road conditions.

The other options do not reflect this requirement accurately. While mud flaps can be optional for some vehicles, trucks with four or more tires specifically require them. The suggestion that mud flaps need to be painted red is not a standard regulation; colors for mud flaps are typically not mandated. Additionally, the notion that mud flaps must cover the rear axle isn't universally applicable, as the primary concern is their effective coverage to prevent debris from being ejected rather than their position in relation to the axle.