What should be noted about exhaust or engine brake retarders fitted to a bus?

Answer: A — They increase engine noise

Exhaust or engine brake retarders are designed primarily to enhance braking performance in heavy vehicles such as buses. One key aspect of this type of braking system is that it indeed increases engine noise. This noise results from the engine's exhaust system mechanism, which activates to assist in slowing down the vehicle by using the engine's own output to create resistance. When the retarder is engaged, it allows for a safer descent on inclined roads and minimizes wear on traditional brake components, which can become overheated and worn down with excessive use. Considering the other options, while it is true that engine brake retarders might have certain conditions of effectiveness, stating that they work only under hard braking or should be reserved for emergencies does not reflect the full range of their intended utility. They can be used regularly during normal operations, such as going downhill, to help manage speed without relying solely on foot brakes. Therefore, the acknowledgment of increased engine noise is an accurate and relevant characteristic of exhaust or engine brake retarders in buses.