When a piece of stone tied to a string is made to revolve in a circular orbit and the string breaks, the stone will continue to move along a tangent to the circle. This means that option 3, "move along a tangent," is the correct answer.

To understand this concept, we can consider the forces acting on the stone while it is in circular motion. When an object moves in a circular path, it experiences a centripetal force directed towards the center of the circle. In this case, the string provides the centripetal force that keeps the stone in circular motion.

However, when the string breaks, there is no longer a force pulling the stone towards the center of the circle. As a result, the stone will continue to move in a straight line along the tangent to the circle at the point where the string broke. This is because an object in motion tends to stay in motion in a straight line, according to Newton`s first law of motion.

Therefore, when the string breaks, the stone will move along a tangent to the circular orbit, as stated in option 3.