Newton's third law of motion states that for every action there is an equal and opposite reaction. The balloon travels in the opposite direction as the air escaping from it. So when gas is released from the balloon it pushes against the outside air and the outside air pushes back. As a result of this the rocket is propelled forward by the opposing force. This opposing force is thrust.
Hope this helps!
Can u plz mark me as brainliest? I really need it!