In a marine water-tube boiler, which component preheats incoming feedwater by recovering heat from the flue gases leaving the generating tube banks?

Why C is correct

The economizer is positioned in the flue gas path beyond the generating tubes and uses that residual heat to preheat incoming feedwater before it enters the drum. The superheater raises steam temperature above saturation; the air heater warms combustion air; the deaerating feed tank removes dissolved gases but is not a flue-gas heat-recovery surface.