At what maximum RPM are auxiliary retarders most efficient?

Answer: C — 90%

Auxiliary retarders, such as those found in heavy vehicles with air brake systems, are designed to provide additional braking power by utilizing engine or transmission components. They are most efficient at around 90% of their maximum RPM. At this level, the retarder can effectively harness the engine's compression or work to its full potential without experiencing significant losses due to overheating or mechanical stress. When operating at 90% RPM, the retarder can produce sufficient resistance to help slow the vehicle while maintaining optimal performance and safety. Beyond this point, approaching or reaching 100% may lead to diminished returns in performance and increased wear on the system. Hence, 90% RPM represents the sweet spot where auxiliary retarders can maximize braking efficiency and output while minimizing potential risks associated with higher RPMs.