I need to block my opponent's immediate horizontal win threat. They have three Xs in a row at positions (3,2), (3,3), and (3,4). If they place a piece in column 5, it would fall to row 3 and give them 4 in a row. By placing my piece in column 5, it will fall to row 3 and prevent this win. This is the most urgent defensive move, as no other placement offers me an immediate winning opportunity.