The correct answer is D) Freeboard. Freeboard is an indication of reserve buoyancy, which is the additional buoyancy a vessel has beyond the minimum required to support its weight. Adequate freeboard, which is the distance between the waterline and the main deck, is essential for maintaining reserve buoyancy and stability, especially in rough seas. Vessels with insufficient freeboard are more susceptible to taking on water and potentially capsizing. The other options are not direct indicators of reserve buoyancy. Righting moment, metacentric height, and rolling period are related to a vessel's stability, but they do not directly measure the reserve buoyancy available.