In which of the following cases do you get the most effective braking in a bus with a retarder?

The engine brake, also known as a retarder, uses the internal resistance of the engine to reduce the vehicle's speed without using the service brakes. Its effectiveness is directly proportional to the engine speed – the higher the revs, the greater the resistance from the engine and the stronger the braking effect. Therefore, to achieve maximum braking force, the engine should be kept in its upper rev range, which often requires changing down a gear.