I need to block my opponent's immediate threat of winning. Looking at the board, I can see that X has two marks in a vertical column at positions (0,1) and (1,1). If X places their next mark at (2,1), they would complete a vertical line and win the game.

While I could try setting up my own winning opportunity (like playing at 0,2 or 2,2 to create diagonal possibilities), blocking the immediate threat takes priority. By placing my O at (2,1), I prevent X from winning on their next move.

Additionally, this move still keeps my future options open. After blocking at (2,1), I'll still have opportunities to create winning lines in subsequent moves.