In my opinion, it’s “A. teenager”
I was wrong, it’s “C. friends” because of “friends - their parents”
‘their’ refers to the friends. ‘their parents’ being a 3rd person possessive of the (friends as a collective, but their individual parents).
EDIT: It’s a little confusing, but it refers to the ‘friends’.
I guess the question has already been answered, but… am I the only one who finds the phrasing here extremely awkward, even borderline ungrammatical? “A teenager who has friends that do things …” is not even a complete sentence.
their is a pronoun, specifically a possessive adjective. It is taking the place of friends (parents’ friends).
I agree that the question/answer could be a bit confusing, and I usually often find the way they phrase these questionnaires confusing. 