The correct answer is C) low superheater outlet temperature. Insufficient combustion air supply to the furnace would reduce the efficiency of the combustion process, leading to incomplete burning of the fuel. This would result in lower heat transfer to the superheater, causing a decrease in the superheater outlet temperature. The other options are incorrect because: A) high feed water consumption is more likely to be caused by leaks or other issues, not insufficient combustion air; B) high stack temperature is typically a sign of excess air, not insufficient air; and D) the fires sputtering would be an indicator of unstable combustion, not necessarily low superheater outlet temperature.