The correct answer is A) close the fuel supply valve. This is the quickest method to stop a small diesel engine whose throttle or governor has become stuck open. By closing the fuel supply valve, you immediately cut off the engine's fuel source, causing it to stall and stop running. This is a quick and effective way to regain control of the engine without having to turn off the ignition or drain the fuel tank, which would take more time. The other options are less effective or appropriate in this situation. Turning off the ignition switch (B) would not immediately stop a diesel engine with a stuck throttle, as diesel engines can continue running even with the ignition off. Draining the fuel tank (C) is a more involved process that would take longer to stop the engine. Applying the shaft brake (D) may not be a viable option on all small diesel engines.