I’ve examined the board and noted that the center column (column 3) is already partially occupied (with X at row 5 and row 3, and an O at row 4). Dropping an O in column 3 (which will fall to row 2) continues to contest the central columns—which are key in Connect Four—while also setting up future opportunities for both vertical and diagonal connections. It also helps block any potential threats from X in the center. Hence I choose column 3.