The correct answer B. Necessity.
Explanation
Modal verbs are auxiliary verbs used in English to express an opinion about whether something is probable or possible. They are also used for over-ability, to ask permission, or to make a request. The modal verbs are: Can, May, Will, Ought to, Must, Could, Might, Shall, Should to and Would.
In the case of the sentence "You don't look well. You must sit down for a minute" the modal verb must is used to refer to a suggestion to sit down since whoever says the sentence sees as a need that the person sits to make him/her feel better. Therefore, the correct answer is B. Necessity.
