I opt for pos'n C. I always feel most secure on the right. Out in the lane, with cars on my right - as in pos'n A or B, I feel awfully exposed.
But this thread points out one thing - when you're on a bike in traffic, you're in a dangerous situation no matter where you are. At A or B you can get rear-ended - or right hooked for that matter. At C and D you can get left hooked. No matter where you are, you are in danger of being run down if you are not extremely alert at ALL TIMES!
But I'll take C, as it's closest to the sidewalk where I can leap to safety if I have to.