When the ball on the left is pulled to the side and released, it swings back due to gravity and will collide with the ball on the right. Since both balls have the same mass and are in a situation akin to a perfectly elastic collision (assuming no energy loss), the first ball (the one on the left) will transfer its momentum to the second ball (the one on the right).
As a result, the ball on the left will stop after the collision, and the ball on the right will swing to the right. Therefore, the correct response is:
The ball on the right will swing to the right.