The following two are correct:
III. If we had gone earlier, we would have found all the stores open.
V. If I were a boy, I’d turn off my phone.
I want to point out one thing about the fifth option. “If/were” clauses are what are known as “contrary to fact,” and for contrary-to-fact statements, you’ll always need a plural verb regardless of whether or not the subject is plural. For instance, “I” is singular; however, for contrary-to-fact statements (that use “if” or “wish”), in order for there to be subject-verb agreement, the verb will need to be plural regardless of the number of the subject as we see with the fifth sentence.