A ball floats on the surface of water in a container exposed to the atmosphere. When the container is covered and the air is partially removed, then the ball

In the given situation, the weight of the ball is supported by the buoyant force of water and buoyant force due to air. As the density of air becomes less, the buoyant force due to the air becomes less. Therefore, to compensate a greater buoyant force due to water is required which is achieved by a greater immersion of ball into the water.