The correct answer is D) Secure the fuel supply and block the air intake. This is the correct answer because stopping the flow of fuel and air is the most effective way to stop an overspeeding diesel engine. Cutting off both the fuel supply and air intake will starve the engine of the necessary components to sustain combustion, causing it to stall and stop running. The other options are less effective. Disconnecting the battery cables would stop the engine from starting, but would not necessarily stop an already running overspeeding engine. Draining the hydraulic fluid from the governor sump or blocking cooling air to the radiator would not directly address the root cause of the overspeed condition.