The correct answer is B) one percent for each 10°F rise in feed water temperature. The reasoning behind this is that an economizer in a boiler system is used to preheat the feed water before it enters the boiler, which can result in significant fuel savings. The general rule of thumb is that for every 10°F rise in feed water temperature, the boiler fuel consumption can be reduced by approximately 1%. This is because less energy is required to heat the feed water to the desired temperature, leading to a more efficient boiler operation and fuel savings. The other options are incorrect because they do not accurately reflect the standard relationship between feed water temperature rise and boiler fuel savings. Option A) is too low, option C) is too high, and option D) does not match the typical 10°F per 1% rule.