The correct answer is A) Shut down one pump. When two ballast pumps used for deballasting a single tank start cavitating, the appropriate action is to shut down one of the pumps. Cavitation occurs when the pumps are not able to adequately draw in the required amount of liquid, leading to the formation of air bubbles and a loss of pump performance. Shutting down one of the pumps will reduce the overall demand on the system, allowing the remaining pump to operate more efficiently and avoid further cavitation. The other options are incorrect because closing valves or opening valves would not address the root cause of the cavitation issue, which is the excessive demand on the system. Shutting down one pump is the most direct and effective way to resolve the cavitation problem.