Notice the kernel of a discovered attack for Black: the bishop in front of the rook on the f-file. If the bishop moves, the rook attacks White's king; that makes this a case of discovered check, giving the bishop two moves to make trouble. It’s a light-squared bishop, so look for a good White piece on a light-colored square that it could attack—and find the queen. The bishop needs to attack it in one move and take it with the next. The answer thus is Bxc2, again winning the queen either by BxQ, or (if the queen moves to block the check—an important possibility to remember) with RxQ.