The correct answer is C) decrease in fuel consumption per horsepower-hour. An increase in the air inlet manifold pressure of a diesel engine will result in a greater mass of air being inducted into the cylinders. This increased air supply allows the engine to burn more fuel, resulting in higher cylinder pressures and temperatures. This in turn improves the engine's combustion efficiency, allowing it to produce more power while consuming less fuel per horsepower-hour. The other options are incorrect because: A) an increase in air inlet pressure would increase, not decrease, the maximum cylinder pressure; B) ignition lag would decrease, not increase, with the improved combustion; and D) exhaust manifold pressure would increase, not decrease, due to the higher cylinder pressures.