The correct answer is D) Increase the air density. Aftercoolers are installed in the diesel engine air intake system to cool the air after it has been compressed by the turbocharger. This cooling process increases the air density, allowing more air to be packed into the engine's cylinders. Increased air density leads to a greater mass of air per cylinder, which in turn allows for more complete combustion and increased engine power output. The other options are incorrect because: A) Decreasing air density would reduce engine power; B) Increasing exhaust temperature is not the primary function of aftercoolers; and C) Aftercoolers do not directly affect lube oil temperature.