C, B=D, A.
Work Step by Step
The ball is moving fastest at point C because it has dropped the farthest from its starting point. The ball is moving equally fast at points B and D because the ball has dropped an equal amount from the starting point. (We ignore friction, which would make the ball slightly slower at D than at B). The ball is moving most slowly at point A where it has dropped just a small distance from its starting point.