I like A.  Make sure you go deep with your concrete and heaving of the threshold won't be a problem.  I'm assuming your building won't be heaving or sinking either, but your header will likely sag some over time.  Having a C or a T inside the bottom of the door means your vertical clearance adjustment is more critical.  Only downside to option A is there is no protection from the door blowing out once you leave the center post.  What about a flat on edge (say 3/8 by 6") with about 1 1/2'' exposed to go up into the bottom of the door?  Similar to option E without the horizontal top.  Less likely to hook an implement on it and less dependent on perfect door adjustment.  I might even leave a few spaces along the run to allow trapped water to drain out.