A. You win the point, because your opponents previous shot, the drop-shot which bounced back over to his side of the court, was an illegal shot.
B. Nobody wins the point. Its a let. Whenever a hit ball bounces on one side of the net and goes back to the other side of the net without being hit back, the point is played over.
C. You win the point, because your opponent hit the ball out.
D. Your opponent wins the point, because you failed to hit the ball back to his side of the net when he hit the drop shot.