The correct answer is B) metacentric height. The metacentric height (GM) is the critical initial stability parameter that determines a vessel's initial stability. It is the distance between the metacenter (the point around which the vessel initially rotates when heeled) and the center of gravity of the vessel. A positive GM indicates the vessel has adequate initial stability, whereas a negative GM indicates the vessel is unstable. The other options are incorrect because they do not directly represent the key initial stability parameter. Option A refers to the height of the metacenter above the keel, which is not the same as the metacentric height. Option C is the height of the center of gravity above the keel, and option D is the height of the center of buoyancy above the keel, neither of which are the same as the metacentric height.