The correct answer is C) Shut the steam valve at once, open the drain valves until all moisture is drained, shut the drain line valves, and slowly open the steam valve again. This is the correct answer because water hammer occurs when a valve is opened too quickly, allowing steam to rapidly condense into water and create a dangerous pressure surge. The proper response is to immediately shut the steam valve, open the drain valves to release the accumulated moisture, and then slowly reopen the steam valve to avoid further water hammer. The other options are incorrect because increasing the speed of opening the valve (option B) would only exacerbate the water hammer, and continuing to open the valve while draining the line (options A and D) does not properly address the root cause of the problem.