I'm choosing column 5 because it's a critical defensive move. My opponent has three Xs in a row at the bottom (columns 2, 3, 4) and could win horizontally if they play in column 5. Additionally, there's a diagonal threat forming with Xs at (2,2), (3,3) that could extend to (4,4) and (5,5). By playing in column 5, I block both threats at once. This is clearly the most urgent defensive move on the board.