The correct answer is C) boiler mud drum. The bottom blow valve on a water-tube boiler is typically attached to the boiler mud drum, which is the lowest part of the boiler. The mud drum collects any sediment or impurities that settle out of the boiler water, and the bottom blow valve allows this sludge to be periodically drained from the system. The other options are incorrect because: A) the steam and water drum is at the top of the boiler, B) the external downcomers are not part of the boiler itself, and D) the floor tubes are not the typical location for the bottom blow valve.