The correct answer is A) metacenter. When a vessel is inclined with no environmental forces acting on it, the center of gravity of the vessel will be vertically aligned with the metacenter. The metacenter is the point where the vessel's initial vertical axis intersects the new vertical axis when the vessel is inclined. This alignment occurs because without external forces, the center of gravity remains fixed relative to the vessel's structure. The other options are incorrect because the longitudinal centerline (B), original vertical centerline (C), and center of flotation (D) do not represent the correct point of alignment for the center of gravity in an inclined vessel with no environmental forces.