The correct answer is D) check the cylinder exhaust temperature frequently. The reason this is the correct answer is that monitoring the cylinder exhaust temperature is the most practical way to detect an overload in a single cylinder of a large, low-speed main propulsion diesel engine. Measuring the exhaust temperature of each cylinder can reveal if one cylinder is running hotter than the others, indicating an overload condition in that cylinder. This is a quick and easy way to identify the problematic cylinder without having to isolate and inspect each one individually. The other answer choices are not as practical or effective. Listening for combustion knock (A) may not always be apparent, isolating each cylinder (B) is time-consuming, and checking for black smoke (C) is a less reliable indicator of a single cylinder overload.