The correct answer is D) Sulfur. Sulfur is a major source of boiler corrosion because when fuel oil containing sulfur is burned, it can form sulfur dioxide (SO2) and sulfur trioxide (SO3). These sulfur compounds can then dissolve in water condensate and form sulfuric acid, which is highly corrosive to boiler metal components. The other options are incorrect because carbon, oxygen, and hydrogen are not directly responsible for boiler corrosion in the way that sulfur compounds are. While these elements are important components of fuel oil, they do not contribute to the formation of the corrosive sulfuric acid.