The correct answer is D) Damage to the furnace refractory. When fuel oil is contaminated with salt water, the burning of this mixture can produce glassy slag that melts and runs over the furnace wall. This glassy slag can damage the furnace refractory, which is the heat-resistant material lining the inside of the furnace. The corrosive nature of the salt in the slag can erode and degrade the refractory, leading to damage and potential failure of the furnace. The other options are incorrect because: A) A protective coating is not formed, rather the slag damages the refractory; B) Cracks in the furnace floor are not a direct result of this condition; and C) Increased furnace temperature is not a consequence of the glassy slag formation.