Fill in the blanks with the correct modal
There are plenty of fruits in the fridge. You _____ buy any.

needn't

The correct modal to fill in the blank is needn't. The modal needn't is used to express permission or lack of necessity, and it is the most appropriate modal to use in this context because it conveys the idea that there is no need to buy any fruit. There are already plenty of fruits in the fridge, so there is no need to buy any more.

The other options are incorrect. The modal mustn't is used to express prohibition, but it is too strong in this context. The modal shouldn't is used to express advice or a suggestion, but it is not as strong as needn't. The modal must is used to express obligation, but it is not used to express lack of necessity.