Totally depends on the scenario, I use all 3. If we’re talking about music, hobbies, personal interest type stuff, it’s usually C, without the “tell them you already know”. I try to show that I know instead of telling. Like if someone’s telling me about a band I already know about, I’ll let them finish and then say something that requires deep understanding of the source. If the discussion is academic, B, but like in a respectful way? Not ‘cutting off’ the other person, more so ‘jumping in’. And that’s so we can both establish what we both know and move on to the more interesting stuff. A is used for authority figures that I have no respect for. I have nothing to say to marching orders.