I'm in the same boat as Druski. Normally I have no problem with math but statistics are another story.
I understand the chances, your first pick you have a 1/3 chance and your second pick you have a 1/2 chance. Where I get lost is since one donkey is taken away, why does your first pick not automatically go to a 1/2 chance?
In other words once there are only 2 doors left, why do you have to re-pick to have your chances go up? If there are only 2 doors left and you have picked one, your chances are now 1/2 weather you re-pick or not.
I say option C. Even though the chances have changed, they changed when one door was eliminated, not when you re-pick.