off

寝る前に電気を消してください。 Please ＿＿ ＿＿ ★ ＿＿ the lights before you go to bed.

The correct order is 'Please turn off the lights before you go to bed.'. The 3rd word (★) is 'off'.

正しい語順は Please turn off the lights before you go to bed. (turn off「〜を消す」) となります。3番目の空白（★）には off が入ります。