详解
Choice C is the best answer. In the first paragraph of Passage 1, Douglas claims that Lincoln considers the Constitution to be "a house divided against itself," due to its provisions for the division of the nation into slave states and free states, and to be "in violation of the law of God." In Passage 2, Lincoln objects to this characterization of his position and devotes the majority of the passage to clarifying that it isn't the Constitution he finds fault with, or even its provisions for slavery, but rather with attempts to spread slavery to regions where it isn't currently practiced. Therefore it can be said that a central tension between the two passages arises from, on the one hand, Douglas's criticism of Lincoln for finding fault with the Constitution and, on the other, Lincoln's insistence that Douglas has misrepresented his position.
Choice A is incorrect because Douglas (Passage 1) proposes no changes to federal policies on slavery and because Lincoln (Passage 2) doesn't consider whether changes to such policies would enjoy popular support. Choice B is incorrect because Douglas (Passage 1) never expresses concern about the potential impact of abolition on the US economy and because Lincoln (Passage 2) neither discusses such an impact nor dismisses concerns about it. Choice D is incorrect because neither passage offers any interpretation of federal law.