The correct answer is A) an overheated starting air supply pipe. A leaking air starting valve on a diesel engine with direct-cylinder admission air starting would cause the air supply pipe to overheat. This is because the leaking valve would allow the compressed air to escape, resulting in excessive air consumption and a rise in the pipe temperature. The other answer choices are incorrect because they do not directly relate to a leaking air starting valve. Excessive smoke, carbon deposits, and early fuel ignition could be caused by other engine issues, but are not the primary symptoms of a leaking air starting valve.