The correct answer is C) prevent steam and water flow reversal from the drum should an economizer casualty occur. The purpose of the check valve located between the economizer and the steam drum is to prevent reverse flow in the event of an economizer failure or casualty. This ensures that steam and water do not flow back from the drum into the economizer, which could lead to damage or other operational issues. The check valve acts as a one-way valve, allowing flow in the proper direction from the economizer to the drum, but preventing reverse flow. The other answer choices are incorrect because they do not accurately describe the primary function of the check valve in this specific location. While they may describe general purposes of check valves in other parts of the system, they do not apply to the specific scenario of the economizer-to-drum connection.