the third choice is correct: "an informal tone, because her purpose is to inform young children." this correctly identifies her audience, for one. two of the answer choices mention "parents and teachers" which are irrelevant in this case, because she's creating the presentation for the students, not the teachers/parents. the first answer choice lists her purpose as "frightening" the young children, which is entirely incorrect--she doesn't want to scare them, she wants to explain to them in a slightly informal way, because children wouldn't really understand "formal" language.
