The correct answer is D. Because an appositive phrase is usually separated from the rest of the sentence with the use of commas, just like in this case. And it refers to the noun, the subject. In this case, the appositive phrase is 'the youth council president', and it refers to, or renames 'Stephanie'.
Christopher McCandless was detaching, separating from society, he bonded with people but he didn't want to feel attached. He tells Ronald Franz that they will talk about it when he, Chris, comes back from Alaska. He didn't refused openly to the idea, but didn't commit either.
