Number 5 is nearly correct; you need leur here, as it means them (referring to the parents) as an indirect object pronoun. An indirect object pronoun is used when the verb doesn’t take a direct object (in other words, someone that something is done to or for). In this case, the subject is writing to her parents, which would be écrire à.

Therefore:

Non, elle ne leur a pas écrit.

The only proper way to answer is “non, elle ne leur a pas écrit”.