When a ball is at its highest point, its acceleration is 0. The only thing pulling it down, is gravity so, the correct option is D. Downwards.
I'm not sure however due to the fact that the arrows on the other options are facing backwards meaning the ball would go backwards; and for the one option the ball is going up. However, the ball isn't going backwards nor up, so it must be D. If I am wrong I greatly apologize.
