When a piece of ice floating in a glass of water completely melts, the level of water in the glass remains unchanged. This phenomenon can be explained by the principle of buoyancy and the displacement of water. A floating ice cube displaces a volume of water equal to the weight of the ice. When the ice melts, it turns into water and occupies the same volume that was initially displaced. Since the density of ice is less than that of water, the melted ice, now in liquid form, fills the exact volume it previously displaced. Therefore, the overall water level in the glass does not change. This principle holds true for any ice floating in a liquid, assuming no additional factors such as temperature changes significantly affecting the water’s volume. Hence, the correct answer is [C] remain unchanged. Understanding this concept illustrates fundamental principles of physics and fluid dynamics.