ANS:C - when the stone is at the lowest position

1. Centripetal Acceleration: At any point in the stone's circular motion, it requires a centripetal force directed towards the center of the circle to keep it in its circular path. This centripetal force is provided by the tension in the string. At the lowest point of the vertical circle, the stone has the highest speed and therefore requires the highest centripetal acceleration to maintain its circular motion. As a result, the tension in the string must be sufficient to provide this maximum centripetal force.
2. Gravity: At the lowest point of the vertical circle, the stone is also subjected to the gravitational force pulling it downwards. The tension in the string must not only provide the centripetal force but also counteract the gravitational force acting on the stone. Since the stone's weight (due to gravity) is at its maximum at the lowest point, the tension in the string must also be at its maximum to counteract this gravitational force effectively.