Change the voice: Close the windows before you go to bed.

The windows were being closed before bed.

The windows must be closed before you go to bed.

The windows are closed before you go to bed.

None of these

The sentence is an imperative (command), and in passive voice, "must be" is commonly used for giving instructions.

"Close the windows" changes to "The windows must be closed" to keep the intended meaning and tense.