Final answer:
Stephen Douglas supported the expansion of slavery into new territories under the principle of popular sovereignty, as evidenced by the Freeport Doctrine, however, his stance caused a split within the Democratic Party. Option A is correct.
Step-by-step explanation:
Stephen Douglas's stance on slavery, as reflected in his speech and actions during the debates with Abraham Lincoln, is complex. While Douglas supported the extension of slavery into the territories, he believed in the principle of popular sovereignty.
During the second debate in Freeport, Illinois, he articulated the Freeport Doctrine, where he proposed that residents of a territory could ultimately accept or reject slavery by enacting local police regulations. This position highlighted a rift within the Democratic Party, as it managed to alienate Southern Democrats who wanted stronger protections for slavery.
Douglas's nuanced position on slavery aimed to allow for local control over the issue, but also sought to uphold the principle of popular sovereignty, even if this sometimes clashed with the Dred Scott decision or the wishes of pro-slavery factions.