My first instinct was A, at the base of the neck. But now that I think about it I think I agree with this more. I think it could be argued that at the joint is where the neck really begins, and that the narrow part beneath that is still part of the body. And I think it would look better (and more professional!) if our weevil friend wore his tie there.

Ties are worn around the base of the neck, and the neck is the flexible thin part that connects the head. I see position A as being well below the flex point, which would be like wearing the tie low on the shoulders. That's why I would prefer it at the bottom end of the joint, position C. One could reasonably argue that anything above where the body narrows down towards the neck is part of the neck, in which case A would also make sense.

Semantics on where a neck starts aside, position B is clearly at the top of the neck and is therefore just nonsense not even worth considering.

Also position C lets the tie hang neatly down the front of the body as it should, rather than dragging the ground or dangling loosely in midair.