Please do not open window. [Syntax]

The correct answer is `で` (de). This particle is used with the negative `ない` form of a verb (`あけない`) to create a polite negative request: `~ないでください` (Please don't ~). So, `あけないでください` means "Please don't open." `あけ` is the verb stem, which needs `ない` before `でください`. `ない` is already present before the star. `ください` is already present after the star. *Grammar Tip:* To politely ask someone not to do something, use a verb's `ない` form + `でください` (e.g., 食べないでください - Please don't eat).

正解は「で」。「～ないでください」で丁寧な禁止。「あけ」は動詞語幹。「ない」は既にあり。「ください」も文末。 文法：「ない形」＋「でください」で「～しないでください」。