I'm choosing column 6 because it blocks a potential diagonal threat where my opponent has Xs at (3,1) and (4,2). If I don't block position (6,4) now, my opponent could eventually create a diagonal connect four through positions (3,1), (4,2), (5,3), and (6,4). This is a defensive move that prevents a future winning opportunity for my opponent while not enabling any immediate threats with their next move.