My suggestion...go for Girl C.
Seriously, if you have had a good friendship with Miss A and have always seen her get hurt by other jerks, you should be there for her. Be a friend with the possibility of becoming more. The best woman to marry is a woman who is your best friend.
It sounds like the same situation I had once. I had always hoped I could be with this girl in college who was my best friend, but she had a jerk of a boyfriend and I had a jerk of a girlfriend. We both got dumped on the SAME DAY by the jerks and we started hanging out pretty much constantly. We became better than best friends and eventually got married. We just had our 4th wedding anniversary and it's been great, we still consider each other our best friend too.
Even if it doesn't work out that you can become more than friends, at least you've kept a great friend. Giving up the dating scene for someone you KNOW you're great with is always less troubling than trying to find someone through dates or at bars.
Good luck with whatever you decide.