The gravitational attraction between the Sun and the Earth is exactly modeled by the tension in the string. (C)
This is an excellent demonstration ! It shows:
-- If she uses a heavier ball (more mass), she gets more tension in the string, just like planets.
-- If she wants the ball (planet) to revolve faster, she has to increase the tension in the string.
-- If she lets out a longer string, the ball moves slower, and there's less tension in the string, just like planets.
-- The ball stays in the same plane all the way around, just like planets.