A student swings his keychain around in a circle. What is the source of the centripetal force for this action?

A. tension

B. gravity

C. friction

D. weight

The centripetal force is the force that's causing the object to accelerate on a circle. There are three forces acting on the string: gravity (downward), friction (resisting acceleration but negligible), and tension (force the string applies to the key). The only force that is relevant is tension, and therefore, your answer is A, tension.