takes for granted that some historical work that embodies prejudices is written by historians who
Why this is right
Since this says takes for granted / presumes, we can ask ourselves whether this was a Necessary Assumption the argument depended on. Was the author assuming that some historical work that embodies prejudices was written by historians who claim to be objective? Yes! The author thinks that these many historians who claim/purport to be objective are not. He thinks that some of their work embodies prejudices. That's why he's saying, "we can't accept their proclamations of objectivity". If you negate this assumption it's saying, "There are zero instances of historical work that embodies prejudices written by any historian who proclaims to be objective". That would terribly hurt the author's argument, if not outright contradict the conclusion. Another way to think about this answer is that the author is equating these "many historians who claim to be unbiased" with these other works of history that embodied prejudices. She is assuming that in both cases the historians set out to be unbiased, but they end up with explanations that are skewed by their authors' prejudices. The author is assuming it's a fair comparison. But what if those biased examples the author is citing were written by historians who gladly admit to their bias. They weren't trying to be unbiased? That would make it feel totally unfair to compare these examples of biased history to the "many historians" who are claiming to be committed to unbiased history.
Skill tested: Flaw · how this choice captures the argument's function is the move to repeat next time.