The correct answer is D) Increases boiler efficiency. Blowing tubes, also known as blowing down, is a process where boiler operators periodically open valves to allow steam and water to be expelled from the boiler's water tubes. This accomplishes the goal of increasing boiler efficiency by removing mineral deposits and soot buildup from the internal surfaces of the water tubes. This helps maintain proper heat transfer and prevents reduced steam production, which would decrease the overall efficiency of the boiler system. The other answer choices are incorrect because they do not accurately describe the primary purpose of blowing tubes, which is to enhance boiler efficiency rather than eliminate fuel oil residues, flush mineral deposits, or clean soot from the stack.