The correct answer is B) High superheater temperature. Excessive soot buildup on the economizer would lead to reduced heat transfer, causing the superheater temperature to rise. This is because the economizer is responsible for preheating the boiler feed water before it enters the boiler, and soot buildup on the economizer tubes would inhibit this heat transfer process. As a result, the superheater temperature would be higher than normal, indicating the need for economizer cleaning to remove the soot buildup. The other options are incorrect because they do not directly relate to the condition of the economizer. High feed water temperature (A) and low air temperature (D) would not necessarily indicate soot buildup on the economizer, and lower than usual air pressure in the furnace (C) is more likely to be caused by other issues, such as air leaks or improper air-fuel ratio.