The purpose of retarders is to

Answer: D — Help slow the vehicle while driving to reduce brake wear

Retarders are systems designed to assist in slowing the vehicle down while driving. They help reduce the wear on the brakes by providing additional braking power, especially when going downhill or needing to slow down without relying solely on the brakes. Retarders are particularly useful for heavy vehicles, such as trucks and buses, to enhance braking performance and safety on the road. Therefore, the correct answer is D. Options A, B, and C are incorrect because retarders are not intended to help accelerate the vehicle, steer the vehicle, or maintain speed. Their primary purpose is to aid in slowing down the vehicle to reduce brake wear and enhance overall braking efficiency.