The correct answer is D) keeping lines drained and insulated. Water hammer in steam lines is caused by the formation of pockets of condensed steam (water) in the lines. When the water is suddenly accelerated by the opening of a valve or the starting of a steam engine, it can create a powerful shock wave that can damage pipes and equipment. Keeping the steam lines properly drained and insulated helps prevent the buildup of condensed water and mitigates the risk of water hammer. The other options are incorrect because: A) rapidly opening steam valves can actually exacerbate water hammer; B) replacing elbows with tees does not address the root cause of water accumulation; and C) keeping the steam temperature below saturation point does not prevent water from forming in the lines.