Final answer:
The centripetal force is what keeps the ball moving in a circular path on a horizontal rotating arm, with the tension in the string being the source of this force.
Step-by-step explanation:
The force that keeps the ball moving in a circular path when connected to a horizontal rotating arm is the centripetal force. This force is required for any object in uniform circular motion, as it causes constant acceleration towards the center of the motion's path. In the scenario where a ball is attached to a rotating arm by a string, the tension of the string serves as the physical origin of the centripetal force. According to Newtons second law of motion, the net force, which is mass times acceleration, is directed towards the center of the circle, therefore, the string tension is the centripetal force that keeps the ball on its circular path.