The correct answer is D) increase peak power output. Cooling the intake air supplied to a diesel engine will increase the air charge density, which means more air mass is packed into the engine's cylinders. This allows for a greater amount of fuel to be injected, resulting in a higher combustion pressure and temperature. The increased air charge density leads to a higher peak power output from the engine. The other answer choices are incorrect because: A) Reducing mean effective pressure would actually decrease power output. B) Decreasing the average compression ratio would also decrease power output. C) Decreasing air charge density would have the opposite effect of what the question is asking about.