When inflating a balloon, the process of pushing air into the balloon creates an internal pressure that exerts force on the walls of the balloon. According to Newton's third law of motion, for every action, there is an equal and opposite reaction.
In this context, the equal but opposite force acting on the balloon is the elastic force exerted by the balloon material itself. As you inflate the balloon, the air fills it and pushes outward on the walls, while the balloon's material pushes back inward in response to that pressure. This tension in the balloon's walls is the force opposing the expansion caused by the air pressure inside.
Thus, the correct answer to your question is the elastic force exerted by the balloon's material.