The correct answer is C) waterplane. The waterplane is the enclosed area defined as the intersection of the surface of the water and the hull of a vessel. This is the horizontal plane that intersects the vessel's hull at the water's surface, dividing the vessel into the submerged part below the waterline and the emerged part above the waterline. The other options are incorrect because: A) the baseline is a reference line used for measuring vessel dimensions, B) the longitudinal reference plane is a vertical plane running lengthwise through the vessel's centerline, and D) the amidships plane is a vertical plane perpendicular to the vessel's centerline at the midpoint of the length.