(A) says, "Stuart’s discussions with Abella were one of the means by which West influenced Stuart."
While it's true that the historian does cite Abella as evidence, we still do not know anything about the conversations that Abella and Stuart had with one another. For all we know, Abella could simply be how Stuart and West came to meet. We do not know whether the influence of West traveled through Abella. As a result, we can get rid of (A).
(D) says, "Stuart’s work was not entirely free from West’s influence."
In the stimulus, all of the historian's remarks after the "but" that starts the first sentence serve as evidence that West did, in fact, influence Stuart. (D) captures this, so this is the correct answer.
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