D) May
"must" is a guarantee or demand.
ex. you must be 21 until you can legally drink in the United States.
"is" is a fact or stating something as it appears to be.
ex. He is going on vacation
"will" is saying something is going to happen in the future with reasonable certainty.
ex. When he gets well paying a job, he will buy his own house
"may" is saying something might happen.
ex. He may go to college next year.
hope this helps