fails to establish that something true of some people is true of
Why this is right
When a Flaw answer choice begins with takes for granted / presumes / fails to establish we can ask ourselves, "was the author assuming this?" Was the author assuming "something true of some people is true of only those people". Ugh, yes, we can make this work with our prephrased Objection of, "What if one of Hana's friends bought her the recording? (That would explain how she got that gift without needing to implicate one of her brothers' being present at the party)." Our author, in thinking that it must have been one of Hana's brothers who is responsible for this gift, was assuming that only the brothers knew she wanted that recording, or only the brothers had planned to give her that recording. That's what this answer is getting at. We know that "planned to give Hana this recording" is something true of Hana's brothers. Since the author isn't even considering the possibility that other people also may have "planned to give Hana this recording", he is assuming that "planned to give Hana this recording" is true of only Hana's brothers. Negating this assumption would be saying, "Hana's brothers are not the only people for whom it was true to say they planned to give Hana that recording", and that negation would be a big objection, because it would suggest an alternate explanation the author has failed to consider.
Skill tested: Flaw · how this choice captures the argument's function is the move to repeat next time.