The correct answer B. Necessity.
Step-by-step 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.