When the ball on the left swings back and collides with the ball on the right, the behavior of the two balls will depend on the nature of the collision and the principles of conservation of momentum and energy.
Since the two balls have the same mass, when the left ball strikes the right ball, the left ball will effectively transfer its momentum to the right ball.
Therefore, the correct response is: The ball on the right will swing to the right.
The left ball will stop momentarily after the collision if we assume a perfectly elastic collision in which the left ball comes to rest after transferring all its momentum to the right ball.