I must play at position 2,1 to block my opponent from winning on their next move. The opponent has Xs at positions (0,1) and (1,1), creating a threat to complete a vertical line in the middle column. By placing my O at (2,1), I prevent their immediate win.

While this doesn't create an immediate winning opportunity for me, it's the most critical defensive move. I've also analyzed the other available positions (1,0, 1,2, and 2,0), but none of them creates a guaranteed winning path or a fork (double threat) situation for me, while allowing my opponent to win immediately if I don't block at 2,1.