The correct answer is: May you live long!
The word “may” is used to express a wish or hope. In this case, the speaker is wishing the listener a long life. The word “live” means to exist or to continue to exist. In this case, the speaker is hoping that the listener will continue to exist for a long time.
The word “can” is used to express ability or permission. In this case, the speaker is not expressing either of these things. The speaker is not saying that the listener is able to live long, nor is the speaker giving the listener permission to live long.
The word “may be” is used to express possibility or uncertainty. In this case, the speaker is not expressing either of these things. The speaker is not saying that it is possible or uncertain that the listener will live long. The speaker is expressing a wish or hope that the listener will live long.
The word “how” is used to ask about a way or method. In this case, the speaker is not asking about a way or method for the listener to live long. The speaker is expressing a wish or hope that the listener will live long.
Therefore, the correct answer is: May you live long!