Directions: Supply suitable auxiliaries/idioms/phrases for the question.

I ___________ call my uncle on his fiftieth birthday otherwise he will feel bad.

Here, must is the correct option, as it is a modal verb indicating that somebody is compelled to do something because of a rule or law, or that is necessary or advisable to do something. Might is used to indicate the possibility that something is true or will happen in the future. Would is used to express the sense of will in reported speech or when referring to an event that has not happened yet. Could is used to express possibility.