The correct answer is D) Increase the air density. Aftercoolers installed in the diesel engine air intake system function to decrease the temperature of the air entering the engine. This increased air density allows the engine to intake more air, which results in greater power output. Decreasing the air temperature increases its density, allowing the engine to draw in more oxygen-rich air per intake stroke, improving combustion efficiency and engine performance. The other options are incorrect because: A) Decreasing air density would reduce engine power, not increase it. B) Increasing exhaust temperature would not be the primary function of the aftercooler. C) Decreasing lube oil temperature is not related to the purpose of the aftercooler in the air intake system.