The correct answer is A) a cylinder air charge of higher pressure. Compared to a naturally aspirated diesel engine, a supercharged diesel engine has a higher cylinder air charge pressure. This is because the supercharger increases the air pressure entering the engine's cylinders, allowing more air and fuel to be combusted, resulting in higher power output. The other answer choices are incorrect: B) Reduced cylinder mean effective pressure - Supercharging actually increases the cylinder mean effective pressure, not reduces it. C) Less valve overlap - Valve overlap is not directly affected by the use of a supercharger. D) Reduced blow-by - Supercharging does not inherently reduce blow-by in the engine.