The correct answer is A) permit higher combustion rates. Water wall tubes in a boiler furnace help to increase the heat transfer from the combustion process to the water in the boiler. This allows for higher combustion rates to be achieved, as the furnace walls can absorb more of the heat being generated. The increased heat transfer helps to maintain the desired steam production levels in the boiler. The other options are incorrect because: B) Increasing heat transfer to the mud drum is not the primary purpose of water wall tubes. C) Water wall tubes would increase, not decrease, the flow of gases through the furnace. D) Water wall tubes increase, not decrease, the flow of gases through the furnace.