The correct answer is B) installing fins on the firesides of water-tubes. Installing fins on the firesides of water-tubes increases the surface area of the tubes, which in turn increases the rate of heat transfer from the hot combustion gases to the water in the boiler. This is an effective way to improve the thermal efficiency of the boiler. The other options are incorrect because: A) Operating the boiler at less than normal water level would decrease the heat transfer rate, as there would be less water to absorb the heat. C) Increasing the amount of excess air would actually reduce the heat transfer rate, as the excess air would cool the combustion gases. D) Treating the boiler water with chemical oxygen scavengers would not directly affect the rate of heat transfer.