The correct answer is C.
Metacentric height is the distance between the center of buoyancy and the metacenter. The metacenter is a point located above the center of buoyancy that represents the equilibrium position of a floating object. When an object is floating, the buoyant force acts upwards through the center of buoyancy. The weight of the object acts downwards through the center of gravity. If the object is not perfectly stable, the center of gravity will not be directly above the center of buoyancy. In this case, the object will experience a torque that will cause it to rotate until the center of gravity is directly above the center of buoyancy. The metacentric height is a measure of the stability of a floating object. A higher metacentric height indicates that the object is more stable.
Option A is incorrect because the center of gravity is the point where the weight of an object acts. The center of buoyancy is the point where the buoyant force acts. The metacentric height is the distance between these two points.
Option B is incorrect because the metacenter is the point that represents the equilibrium position of a floating object. The center of gravity is the point where the weight of an object acts. The metacentric height is the distance between these two points.
Option D is incorrect because the free surface is the surface of the water that is in contact with the object. The center of buoyancy is the point where the buoyant force acts. The metacentric height is the distance between these two points.