At the bottom-is the correct one.

When a body is moving in a circle with uniform motion, its linear velocity will change continuously. Since the magnitude of the velocity does not change, the change in the velocity is due to the change in its direction. This change in velocity gives rise to a changing momentum. According to Newton's second law of motion, to bring any change in momentum, a force is required. Therefore centripetal force is required to keep the body in the circular motion.

On rainy days, the friction between the tires of the motorbike and the road decreases as the sliding friction is converted into fluid friction because of a thin layer of water between the surfaces. Since every turn is not banked, the friction is not sufficient to provide the necessary centripetal force to move the motorbike round the turn. For this reason, the motorbike slips at the turns of a road on rainy days.

When a vehicle moves in a turning, a centripetal force is required to prevent it from skidding on. The required centripetal force is provided by friction between the tires and the road. However it is not safe to rely only on friction because this friction reduces when the road becomes wet. When the road is banked at the turnings, the component of the normal reaction in the direction toward the center of the banked road provides the necessary centripetal force. This ensures that driving a...