The correct answer is B) one percent for each 10°F rise in feed water temperature. The reason this is the correct answer is that the use of an economizer in a boiler system can lead to fuel savings by preheating the boiler feed water before it enters the boiler. This preheating results in a decrease in the amount of energy required to heat the feed water to the desired temperature, leading to improved boiler efficiency and fuel savings. The general rule of thumb is that boiler fuel savings from the use of an economizer can amount to approximately one percent for each 10°F rise in feed water temperature. The other options are incorrect because they do not accurately represent the typical fuel savings associated with economizer use. Option A) is too low, option C) is too high, and option D) does not align with the commonly accepted relationship between feed water temperature rise and fuel savings.