The correct answer is C) decrease in fuel consumption per horsepower-hour. Increasing the air inlet manifold pressure of a diesel engine will result in a greater amount of air being forced into the cylinder. This increased air supply allows the engine to burn more fuel, which in turn produces more power. Since the power output increases without a corresponding increase in fuel consumption, the fuel consumption per horsepower-hour will decrease. The other answer choices are incorrect because: A) an increase in manifold pressure would increase, not decrease, the maximum cylinder pressure; B) ignition lag would not be affected by manifold pressure; and D) exhaust manifold pressure would increase, not decrease, with higher inlet manifold pressure.