The correct answer is B) High superheater temperature. A high superheater temperature would indicate excessive soot buildup on the economizer. The economizer is responsible for heating the boiler feed water before it enters the boiler. If the economizer becomes coated with soot, it reduces the heat transfer efficiency, leading to higher superheater temperatures as the boiler has to work harder to generate the required steam. The other options are incorrect because they do not directly relate to soot buildup on the economizer. Low air temperature, lower air pressure, and high feed water temperature can be caused by various other factors unrelated to economizer soot accumulation.