You are correct, the answer is A) center of gravity. Stability is determined principally by the location of the center of buoyancy and the center of gravity. The center of gravity is the point at which the entire weight of the vessel is considered to act, and its position relative to the center of buoyancy is the key factor in determining the vessel's stability. The location of the metacenter, the geometric center of the waterplane area, and the center of flotation are also important factors, but the center of gravity is the primary determinant of stability. The other options are incorrect because they do not directly determine the vessel's stability in the same way as the center of gravity. The metacenter, geometric center of the waterplane area, and center of flotation are all relevant factors, but they are not the principal determinants of stability.