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The minimum height for mud flaps is set to ensure they effectively serve their purpose of minimizing the spray of water, mud, and debris from the tires while a vehicle is in motion. The correct answer indicates that mud flaps should be positioned within 8 inches of the road surface. This height ensures that they are close enough to catch debris and prevent it from being thrown upwards by the tires, thereby protecting other vehicles and pedestrians from potential hazards.

Positioning mud flaps higher could lead to decreased effectiveness, as they may not adequately reduce the amount of material being projected from the wheels. Therefore, the regulation around this measurement exists to promote safety and compliance with road usage standards. Understanding this height requirement is crucial for commercial drivers to maintain vehicle legality and road safety standards.