I'm placing my piece in column 1 because it will block my opponent's immediate horizontal win threat in row 3. They currently have three X pieces in a row at positions (2,3), (3,3), and (4,3). If I don't block column 1, they'll win on their next move. My piece will fall to row 3 in column 1, preventing their connect four. While my opponent still has another threat in row 4 (columns 1-3 threatening column 0), I can only block one threat, and blocking in column 1 is the most direct response to prevent an immediate loss.