When the left ball is pulled to the side and released, it will swing down due to gravity. When it swings back and collides with the ball on the right, a conservation of momentum and energy principle applies. Since both balls have the same mass and are initially at rest, the left ball will transfer its momentum to the right ball upon collision.
During an elastic collision (which is a reasonable assumption for metal balls), the left ball will come to a stop after the collision, and the right ball will begin to move to the right. Therefore, the correct answer is:
a. The ball on the right will swing to the right.