Name the force which is responsible for circular motion.

An object moving in a circle is experiencing an acceleration.
Even if moving around the perimeter of the circle with a constant speed, there is still a change in velocity and subsequently an acceleration.
This acceleration is directed towards the center of the circle and according with Newton's second law of motion, an object which experiences an acceleration must also be experiencing a net force.
The direction of the net force is in the same direction as the acceleration.
So for an object moving in a circle, there must be an inward force acting upon it in order to cause its inward acceleration.
This is sometimes referred to as the centripetal force requirement.
The word centripetal (not to be confused with the F-word centrifugal) means center seeking.
For object's moving in circular motion, there is a net force acting towards the center which causes the object to seek the center.