The correct answer is D) use a fuel having a lower ignition temperature. When restarting a heavy fuel diesel engine that has been stopped for some time, using a fuel with a lower ignition temperature can help facilitate the engine's restart. This is because the engine components may have cooled down, making it more difficult for the fuel to ignite and start the engine. A fuel with a lower ignition temperature requires less heat to ignite, aiding the restart process. The other options are incorrect because they do not directly address the issue of the fuel's ignition characteristics. Increasing the starting air pressure (A), using a higher than normal cranking speed (B), or increasing the fuel injection pressure (C) may not be as effective as using a fuel with a lower ignition temperature in this scenario.