Choice B is the best answer. In the first paragraph of Passage 1, Douglas argues that throughout the period in which the United States had both free and slave states, the nation as a whole "increased from four millions to thirty millions of people . . . extended our territory from the Mississippi to the Pacific Ocean . . . acquired the Floridas and Texas . . . [and had] risen from a weak and feeble power to become the terror and admiration of the civilized world." It can reasonably be inferred that Douglas cites such growth in territory and population to make the point that the division into free and slave states was obviously not a threat to the country's health or survival.
Choice A is incorrect because although it can be inferred that Douglas would argue for continued expansion of the United States, he cites the expansion it has already undergone as support for perpetuating the division into free and slave states. Choice C is incorrect because although Douglas implies that basic facts pertaining to the historical growth of the nation cast doubt on Lincoln's political agenda, he doesn't imply that Lincoln is unaware of those facts. Choice D is incorrect because although Douglas notes that the United States is globally perceived to be powerful, he doesn't imply that this perception can be accounted for by the nation's record of growth.