In the following question, the sentence given with blank to be filled in with an appropriate word. Select the correct alternative out of the four and indicate it by selecting the appropriate option.
She must ______ a great time at her birthday party.
As we have used must before the blank, be (and not is) should follow. Also, the verb should be in –ing form (having)
Thus, the word be having is the correct fit for the blank, making option 1 the correct answer.
Complete Sentence - She must be having a great time at her birthday party.