“Legitimate” documentaries? The legitimacy of a documentary is different from its worth. And, the premises do not establish that there are any documentaries with “no purpose other than authentically depicting the behavior of its human subjects.” We do have enough to suggest that one of the purposes of the author’s documentaries is to depict the behavior of its subjects. But that doesn’t mean it’s the only purpose.

(B) doesn’t help establish the worth of one documentary over another.

(C) has nothing to do with establishing the worth of one documentary over another. If we added (C) to the arguments, we would still have no idea whether the author’s documentaries are more worthwhile.

(D) isn’t phrased in the conditional form that we predicted, but it captures a similar idea. If we add (D) to the argument, a documentary becomes more worthwhile as it depicts its subjects’ behavior more authentically. This supports the reasoning that a documentary that shows natural behavior – like the author’s documentaries – is more worthwhile than a documentary that doesn’t show natural behavior.

(E) presents a rule about what people in documentaries should not try to do. But the argument concerns whether documentaries are more worthwhile or less worthwhile. What people in documentaries should not do has no connection to a conclusion about the worth of a documentary.