You indicated that the angle between the velocity and acceleration at point D is obtuse but not 180°. An obtuse angle smaller than 180° indicates that the ball is slowing down and following a curved path. The ball slows down prior to point D. It speeds up after point D. When the ball is exactly at point D, is it either speeding up or slowing down?

In addition, remember that at point D the ball is flying through a vacuum. Hence, the acceleration of the ball at point D is the freefall acceleration (i.e., the acceleration of gravity). In what direction is the freefall acceleration?