Answer:
Grandma Suzie will be ninety years of age this month.
Step-by-step explanation:
The correct way to complete the sentence above is to use the verb "to be" in its unconjugated form, so the correct thing is to use the word "be", which will act together with the auxiliary verb "will" to signal the age which grandmother will complete in the given month. In this way, the sentence becomes coherent, fluid and grammatically correct.