I must play in column 5 to block my opponent from creating a horizontal line of 4 X's in row 3. My opponent currently has 3 X's in a row (positions 2-3-4) at row 3, and if they play in column 5, they would win immediately. This is a defensive move that prevents an immediate loss. While there are other columns I could play in, none offer an immediate winning opportunity for me, making this blocking move the highest priority.