The correct answer is D) remove settled solids from the water drum. Bottom blow valves are installed on auxiliary water-tube boilers to allow the removal of settled solids and sludge that accumulate in the bottom of the water drum. This helps maintain water quality and prevent buildup that can reduce boiler efficiency and potentially damage internal components. The other answer choices are incorrect because they do not accurately describe the primary purpose of bottom blow valves. Option A is too broad, as bottom blow valves are not used to completely drain the boiler in an emergency. Option B addresses sludge, but in the steam drum rather than the water drum. Option C refers to removing impurities from the water surface, which is not the function of bottom blow valves.