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 water drum at the bottom of the boiler. This helps maintain proper water circulation and prevents the buildup of deposits that could interfere with heat transfer and potentially damage the boiler. The other answer choices are incorrect because: A) Boilers typically have separate emergency drain valves for complete draining, B) Preventing sludge buildup is not the primary purpose of bottom blow valves, and C) Removing floating impurities is typically done through the use of surface blow valves, not bottom blow valves.