The correct answer is B) there is most likely an obstruction in the lower valve. When a boiler is rolling in heavy seas, the water level in the gage glass should remain steady if the gage glass is functioning normally. However, if the lower valve of the gage glass is obstructed, the water level in the gage glass will not accurately reflect the actual water level in the steam drum, even though the water level in the steam drum may be adequate. This obstruction in the lower valve is the most likely cause of the steady water level in the gage glass despite the vessel's rolling motion. The other answer choices are incorrect because A) a steady water level in the gage glass does not indicate a low water level in the steam drum, C) the baffling of the steam drum is not the cause of the steady gage glass reading, and D) the steady gage glass reading actually indicates a problem with the gage glass, not normal functioning.