I chose B, because that is what I have done when this has happened. Except, they knew from the beginning that I was in a loving relationship - so they kept things entirely appropriate, did not flirt or hit on me (but they did like me... they stupidly told my boyfriend this while drunk). I really liked them as a person, but it would just upset my boyfriend to know I was spending too much time talking to or hanging around with guys who liked me in that way. So I put some considerable distance, but stayed friendly.
But if they were actually hitting on me or flirting, it would be C. In fact that has happened before too, I had a friend I liked to talk to, he started flirting with me when he knew I was taken - I started to avoid them completely.
Also: C if they touch me... Generally my personality and the way I talk makes it clear that I'm not the flirty touchy feely kind of person, so everyone just knows not to. If they overstep that, they know what they're doing.