I come across this sort of thing from time to time, and I agree with most of the respondents that B is the best position. But I'd like to point out that B is not always possible, especially in heavy traffic. If you're riding to the right to begin with, you may not be able to get to B if traffic is heavy and no one lets you in (I feel like this happens to me frequently).

That leaves you with C and D. I prefer D because it allows me to see who's turning and who's not. I'll pull up in front of the first car that's going straight, while still being very careful that that car may be intending to turn, but has decided not to signal.