All of these are great, Larry
! I always spend days debating which pieces of my work to enter in a competition, so I know what you're going through.
In my experience, size seems to matter...it seems like the winners often are large paintings/drawings, etc. Even so you should go with your gut feeling and personal favorite.
My favorite of these has got to be C. That tall, proud bison on the cool blue background....it's eye-catching, bold, and really shows your style nicely. If it were my choice, I'd definitely choose that one. I hope this helps. Of course, in the end, it's your choice! Let us know how it goes, and good luck!