A stone is tied to one end of string. holding to the other end , the string is whirled in the horizontal plane with progressively increasng speed it breaks at some speed bexause

Breaks because it doesn't have force in the radial direction and thus due to inertia the stones will fly off tangentially.

Explanation:

Until the stone is tied to a string, it will possess centripetal force and centrifugal force along the radial direction of motion due to the tension present in the string. Now, when the rope breaks, it breaks because it doesn't have force in the radial direction and thus due to inertia the stones will fly off tangentially.