You indicated that the angle between the velocity and acceleration at point D is acute but not 0°.  An acute angle bigger than 0° indicates that the ball is speeding up and following a curved path.  Is the ball speeding up at point D?

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?

Detailed Help Theory