The correct answer is A) Damage to the furnace refractory. The burning of fuel oil contaminated with salt water can lead to the formation of glassy slag, which melts and runs over the furnace wall. This glassy slag can cause damage to the furnace refractory, which is the heat-resistant material lining the inside of the furnace. The high temperature and corrosive nature of the molten slag can erode and degrade the refractory, leading to its deterioration and potential failure. The other answer choices are incorrect because: B) Cracks through the furnace floor are not a direct consequence of the glassy slag; C) Increased furnace temperature is not necessarily a result of the glassy slag formation; and D) The glassy slag does not form a protective coating, but rather causes damage to the existing refractory.