When will any kind of engine brake usually operate at peak efficiency?

The correct answer is that an engine brake usually operates at peak efficiency at rated speed rpm. Engine brakes, such as Jake brakes or exhaust brakes, function by altering the engine's normal operation to create additional braking force. This is most effective when the engine is running at or near its rated speed, where it can generate maximum power and torque, allowing for optimal braking performance.

At rated speed, the engine can utilize its full capacity, taking advantage of the engine's design characteristics, such as valve timing and cylinder pressure, to produce the greatest amount of braking effect. Operating at peak torque rpm or the highest rpm may not be as effective for braking purposes. Idle rpm generally does not produce enough power or torque to create significant braking force, making it an ineffective point for engine brake operation.