Correct option is (2) At the top of a hill the riders experience weightlessness. It means that the centrifugal force acting on the roller coaster must have a magnitude equal to that of mg , which is it's weight. So balancing these forces we get, v²/R = g Here, R = 20m solving for v, we get v = 14.14 m/s Which lies in between 14m/s and 15m/s