Answer
We can not conclude that the wall is charged.
When the balloon is brought close to the wall, the electrons in the wall are pushed away from the balloon, thus exposing the positive charges in the wall. These positive charges and the negative charges in the balloon attract each other. This is why the balloon sticks to the wall.
Work Step by Step
When a balloon is rubbed with wool, electrons move from the wool to the balloon, so the balloon has a negative charge.
When the balloon is brought close to the wall, the electrons in the wall are pushed away from the balloon, thus exposing the positive charges in the wall. These positive charges and the negative charges in the balloon attract each other. This is why the balloon sticks to the wall. Therefore, we can not conclude that the wall is charged.
