The correct answer is B) center of buoyancy. The stability of a vessel is determined principally by the relationship between the center of gravity and the center of buoyancy. The center of buoyancy is the geometric center of the underwater volume of the vessel, and its location relative to the center of gravity is what determines the stability characteristics of the vessel. If the center of gravity is below the center of buoyancy, the vessel will have positive stability and tend to right itself when heeled over. The other options (keel, center of flotation, aft perpendicular) are not directly related to the fundamental principles of vessel stability.