Correct answer:
D. hará
Milagros: Mañana hará mucho calor.
It's common to use the verb hacer when talking about the climate. So here this verb is being used in the simple future that is used we use to talk about what will or shall happen. Therefore, Milagros says that tomorrow will be very hot. The other options are estará (comes from the verb estar), será (comes from the verb ser) and va a (comes from the verb ir plus the preposition a)
