Final answer:
Stephen Douglas is describing the concept of popular sovereignty in the passage. This doctrine gave people in a territory the right to decide on the matter of slavery, leading to debates and conflicts which influenced events leading up to the Civil War.
Step-by-step explanation:
The passage from the speech given by Stephen Douglas is describing B) the concept of popular sovereignty. Douglas argues that regardless of the Supreme Court's decisions, including the Dred Scott ruling, the people in a territory have the legal means to decide whether slavery should be allowed or prohibited. This became known as the Freeport Doctrine, which supported popular sovereignty but caused a rift within the Democratic Party, especially between the Northern and Southern Democrats. Popular sovereignty was an important political doctrine during this time, asserting that the resident settlers of a territory had the democratic right to decide on the issue of slavery within that territory.
The Wilmot Proviso, another term mentioned in your materials, aimed at prohibiting the expansion of slavery into territories won from Mexico, but it failed to pass the Senate. The concept of popular sovereignty and the Wilmot Proviso were both significant in the lead-up to the American Civil War, as they reflected the intense disagreements over the expansion of slavery.