The correct answer is: D. Should
The word “should” is used to express a recommendation or suggestion. In this case, the speaker is suggesting that the person they are talking to should not leave, as they have only just arrived.
The word “must” is used to express a strong obligation or necessity. In this case, the speaker is not expressing a strong obligation or necessity, so “must” is not the correct answer.
The word “can” is used to express ability or permission. In this case, the speaker is not expressing ability or permission, so “can” is not the correct answer.
The word “how” is used to ask for information about how to do something. In this case, the speaker is not asking for information about how to do something, so “how” is not the correct answer.