When a naturally aspirated four-stroke cycle diesel engine is converted for supercharging, which of the following changes must be made to the valve timing?

Correct Answer: Option A — The intake valve opening is advanced and the exhaust valve closing is retarded.

Supercharging increases scavenging and charging efficiency. Converting a naturally aspirated four-stroke engine requires advancing intake valve opening and retarding exhaust valve closing to maximize the valve overlap period, facilitating effective cylinder scavenging and cooling by incoming high-pressure charge air.