Simple odd's, assuming the deer isnt locked into your headlights and frozen...
Trying to guess which way a deer WONT go is 50% odds your going to hit him.. as you can try left or right, you have removed standing still from the equation... in this case, the deer not moving would be the same as further left or right from your decision... (ie if you choose left, and he doesnt move, the deer has decided to move not very far to your right)
choosing to go straight, and giving the deer the choice of left or right suddenly means there is a 33% chance the deer will hit you as there are now 3 possible choices... left, right, or doesnt move...
Straight also gives you maximum braking ability compared to any swerve combined with brakes...
If the deer is not frozen, the correct choice is straight...