Centripetal force is the force acting on the object moving in a circular path with direction towards the center. The centripetal force causes the object to follow a curved path.
Three forces together act on the string with key: Tension (a force acting on the key chain due to string), gravity (the downward force), and friction (resisting force). Out of these the main source of action of centripetal force is the "tension" applied by the string on the key-chain.
