The correct answer is D) metacentric height. The metacentric height (GM) is the distance from the center of gravity (G) to the metacenter (M), which is the point where the vertical line through the center of buoyancy (B) intersects the vertical line through the new center of buoyancy when the vessel is slightly inclined. When small-angle stability applies, the metacentric height is the key stability term that determines a vessel's initial stability. The other options are incorrect because: A) the height of the metacenter (M) is not the relevant stability term, B) the metacentric radius (BM) is a different parameter, and C) the righting arm (GZ) is a different stability characteristic that depends on the metacentric height.