(C) must be
"When it gets really hot, rain must be a blessing" — 'must be' expresses logical certainty/strong deduction, which best completes this slogan.
'Must be' is used to express a logical conclusion or strong deduction based on evidence. Since the context (extreme heat) makes it almost certain that rain would be a blessing, 'must be' is the correct modal verb. 'Should/ought to be' express obligation, and 'will be' expresses simple future — neither fits the emphatic, deductive tone of a slogan.