Answer:
D. don't have to
Step-by-step explanation:
This is because "don't have to" means that it is not necessary or required for someone to do something. In this case, if someone does not feel well, they are not required to come to the party. "Shouldn't" means that it is not advisable or appropriate for someone to do something. "Mustn't" means that it is prohibited or forbidden for someone to do something. "Had better not" means that it is advisable or necessary for someone to not do something. All of these options are incorrect because they do not accurately convey the idea that someone is not required to come to the party if they do not feel well.