The question says, "Which one of the following most accurately expresses the main idea of the passage?"
This is a main point question. You asked why (C) is wrong, so let's take a look at it first.
(C) says, "U.S. historiography in the late nineteenth and early twentieth centuries was characterized by a conflict between African American historians who viewed history from a transnational perspective and mainstream historians who took a nationalistic perspective."
While it's true that the author spends lots of the text discussing the similarities and differences between the transnational and mainstream approaches to historiography, we cannot conclude with certainty that the conflict between these two approaches is how U.S. historiography can be characterized. It is entirely possible that the conflict between mainstream and transnational U.S. historiography represented only a small fraction of all U.S. historiography that historians practiced during this time period. Further, (C) emphasizes U.S. historiography, and that wasn't the main point of the passage; rather, the author's central point is about African American historians. Based on these two things, we can get rid of this answer choice.
(D) says, "The transnational perspective of early African American historians countered mainstream nationalist historiography, but it was arguably nationalist itself to the extent that it posited a culturally unified diasporic community."
In reading the passage, the author's main point is revealed in the last paragraph in lines 43-45 ("Yet, for all their distrust of U.S. nationalism, most early black historians were themselves engaged in a sort of nation building") and lines 50-55 ("Thus, one might argue that black historians' internationalism was a manifestation of a kind of nationalism that posits a diasporic community, which, while lacking a sovereign territory or official language, possesses a single culture").
The rest of the passage works toward supporting this main idea, so we know the main point of the passage is to show that African American historians opposed a national approach to historiography even though they were simultaneously involved with their own type of nationalism. This is what (D) says almost verbatim, so it's the correct answer choice.
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