Choice A is the best answer. In the first paragraph of Passage 1, Douglas discusses the issue of slavery in the context of the division of free states and slave states throughout the period when the United States "extended our territory from the Mississippi to the Pacific Ocean" and "acquired the Floridas and Texas, and other territory sufficient to double our geographical extent." In the second paragraph of Passage 2, Lincoln asserts that the controversy over slavery has historically been "excited by the effort to spread [slavery] into new territory," as in the case of Missouri, Texas, and "the territory acquired by the Mexican War." Therefore, it can be said that, notwithstanding their differences of opinion, both Douglas and Lincoln discuss the issue of slavery in relationship to the expansion of the Union.
Choices B, C, and D are incorrect because it is in relationship to the nation's expansion that both passages discuss the issue of slavery, not in relationship to questions of morality (choice B), religious toleration (choice C), or laws regulating commerce (choice D).