The purpose of retarders in heavy vehicles is to:

The correct choice is focused on the primary function of retarders in heavy vehicles, which is to assist in slowing the vehicle and reducing the wear and tear on the brakes. Retarders work by providing additional resistance against the vehicle's motion, which helps control speed, especially on declines or during braking situations. This resistance is mainly generated through the vehicle’s drivetrain or engine, which slows the vehicle down without relying solely on the conventional brake system.

Using retarders reduces the need to apply the brakes frequently, thus minimizing heat buildup and wear on brake components. This can significantly extend the lifespan of the braking system, leading to lower maintenance and replacement costs over time. In heavy vehicles, where brake performance is critical for safety and control, the use of retarders is an essential feature for effective vehicle operation.

The other options do not align with the primary role of retarders. For example, retarders do not increase speed, enhance engine performance, or directly improve fuel efficiency, although by reducing brake usage, they can indirectly contribute to longer vehicle life and potentially better overall efficiencies.