The correct answer is A) lighting off a cold ship. When a steam plant is lit off from a cold condition, diesel oil is often used to provide the initial fuel to the burners. This is because diesel oil is easier to ignite and burn than heavy fuel oils, which require higher temperatures and pressures to atomize and combust effectively. Using diesel oil during the lighting off process helps ensure a reliable and controlled startup of the steam plant, especially for a cold ship. The other answer choices are incorrect because they do not directly relate to the specific scenario of lighting off a cold steam plant. Diesel oil would not typically be used for heavy smoking, overload capacity, or blending heavy fuels, which have different operational requirements compared to a cold startup.