The correct answer is C) excessive air. Explanation: An excessively high superheater temperature is typically caused by excessive air in the boiler system. When there is too much air, it leads to incomplete combustion, which in turn causes the superheater temperature to rise excessively. This is because the excess air lowers the temperature in the furnace, requiring the superheater to work harder to achieve the desired steam temperature. The other options are incorrect because: A) soot accumulation would reduce heat transfer and lower superheater temperature, B) high feed water temperature would not directly cause excessive superheater temperature, and D) excessive steam demand would increase the load on the superheater but not necessarily lead to excessively high temperatures.