B is telling the truth.
Work through it: assume each person is the sole truth-teller and check for consistency.
If A is the truth-teller, then B is lying, so B's claim ("C is lying") is false, meaning C tells the truth — but then we'd have two truth-tellers (A and C). Contradiction.
If C is the truth-teller, then both A and B are lying. B lying means C is not lying, which is consistent so far. But A lying means B is not lying — contradiction, since we just said B is lying.
If B is the truth-teller, then C is lying, so it's false that both A and B are lying — meaning at least one of them tells the truth, which is B. That's consistent. A is lying, so B is not lying — also consistent. Everything checks out.
B is the truth-teller. A and C are both lying.