When a ship sailing in a river reaches the sea, the level of the ship will fall slightly (C). This occurs because seawater is denser than river water. As the ship transitions from less dense river water to denser seawater, the buoyant force decreases slightly, causing the ship’s level to lower. Options (A), (B), and (D) are incorrect. The ship’s level cannot remain the same (A) or rise slightly (B) because of the denser seawater. Option (D) is incorrect because the change in the ship’s level is primarily due to the change in the surrounding water’s density, not the weight lying in the ship. Therefore, the correct option is (C) fall slightly.