When the ball on the left is pulled to the side and released, it will swing down due to gravity and collide with the ball on the right. Since both balls have the same mass, we can apply the principles of conservation of momentum and energy in elastic collisions.
When the left ball swings back and hits the right ball, it will transfer its momentum and energy to the ball on the right. As a result, the ball on the left will come to a stop (or move very little depending on the exact dynamics), and the ball on the right will swing to the right.
Thus, the correct option is D: the ball on the left will stop, and the ball on the right will swing to the right.