The correct answer is D) With the engine stopped, cover the air inlet after removing the filter. When cleaning the air filter on a diesel engine equipped with a turbocharger, it is important to take precautions to prevent debris from entering the turbocharger. Covering the air inlet after removing the filter helps prevent any foreign particles from being drawn into the turbocharger, which could potentially damage the sensitive components. The other options, such as reducing engine speed or using compressed air, do not adequately address this risk. Soaking the filter in kerosene is also not recommended, as this could introduce additional contaminants into the engine air intake system.