Answer:
Static electricity.
Explanation:
Electric Charge; Is the quantity of electricity held in an object
The balloon is electrically neutral, meaning it had an equal amount of positive and negative charges hence it couldn't attract the paper.
After rubbing the balloon, it gained an electric charge; and after gaining this charge it got the ability to exert a force on the pieces of paper and attract them, This is called static electricity.
Therefore, Static electricity caused the paper to stick to the balloon.