The problem with (D) is that it's too weak of an answer choice, as we know the author disagrees with the reasons advanced for the restriction on alienability in the Dawes Act at the time of its passage. Thus, (D) is out.
(E) says, "highly skeptical"
(E) does a great job in highlighting the author's blatant disagreement with the reasons. The author doesn't believe that they were the real reason for the passing of the Dawes Act. (E) is the correct answer.
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