The correct answer is D) feed water temperature is too low. When operating under constant load, the superheated steam temperature may rise above normal if the feed water temperature is too low. This is because the lower feed water temperature requires more heat input to the boiler, leading to an increase in superheated steam temperature. The other options are incorrect because: A) excess air is too low would lead to incomplete combustion, not a rise in superheated steam temperature; B) feed water temperature is too high would actually decrease the superheated steam temperature; and C) boiler is priming would cause moisture carryover, not a rise in superheated steam temperature.