You are single if you are not married. This is the legal definition. There are, of course, de facto relationships which are tantamount to marriage but there is no clear definition of such. Some say that jewelry must be exchanged but I firmly believe that the American man, at least, is no longer single if he sleeps in a bed with a pillow sham, dust ruffle, or duvet cover that his girlfriend has gifted to him. There's no going back after that.

At least here in Australia, if asked people would say A. B seems valid, just don't use 'boy friend' to refer to another friend who is a man when you're talking to a potential boyfriend or he'll soon become disinterested in you. C also could be correct if you've got that kind of savoir faire and are a talented liar. But more than anything, be sure to effectively communicate to whoever you're in a relationship with so you can better self disclose, resolve doubts and uncertainties, define boundaries, set goals, and better understand the kind of relationship both parties want.

A. Single means you are not seeing or dating anyone. Regardless of being married or not