The correct answer is B) dew point temperature of the stack gas. The minimum feed water inlet temperature to a boiler economizer is determined by the dew point temperature of the stack gas. This is because the economizer is designed to recover heat from the hot exhaust gases before they exit the boiler. If the feed water inlet temperature is below the dew point of the stack gas, the water vapor in the gas will condense, leading to corrosion issues in the economizer. Therefore, the feed water inlet temperature must be maintained above the dew point temperature to prevent this problem. The other options are incorrect because they do not directly determine the minimum feed water inlet temperature to the economizer. The superheater outlet temperature, surface area of the third stage heater, and radiant heat transfer in the furnace are not the primary factors that influence the economizer's feed water inlet temperature.