Since there's no answer so far, I'll give it a try.
Assuming you at least approximately know the coordinates of the circles (which you do, if the scan is good), I would just count non-white pixels or calculate some sort of color average within a certain area around them. The one with the most non-white pixels or with the "highest blackness" should be the chosen answer then.
With "non-white" pixels I mean, that the difference of the pixel to white is bigger than a certain treshold (because in the scan, the pixels are not perfectly white).
You could also segment the picture into black and white first, that would make the recognition easier/better, I guess.
Then again, this is just an idea.
