Are engine brake retarders more effective at low engine rpm or high rpm?

Answer: B — High engine rpm

Engine brake retarders work by using the engine's compression to slow the vehicle down, so they are most effective at high engine rpm when there is more compression. At low engine rpm, there is less compression to use, making the retarders less effective. Option A is correct because it indicates that low engine rpm is not the most effective, while options C and D are incorrect as they suggest there is no difference or that engine rpm does not affect the retarders at all.