Answer
The middle hill.
Work Step by Step
In all three cases, the speed increases.
Recall Galileo's result (page 46); on a horizontal plane, at an inclination of 0°, the acceleration is zero. On a vertical plane, at 90°, the acceleration has a magnitude of g, the freefall acceleration.
On the left hill, the slope and acceleration are constant.
On the right hill, the acceleration increases because the hill becomes steeper.
On the middle hill, the acceleration decreases toward zero as the ball rolls, because the hill becomes less steep.
