Final answer:
Senator Lewis Cass proposed popular sovereignty, C) allowing territory residents to vote on slavery, a concept leading to strife like 'Bleeding Kansas' and eventually deemed ineffective in resolving conflicts over slavery's expansion.
Step-by-step explanation:
Senator Lewis Cass of Michigan proposed the doctrine of popular sovereignty, a measure that would allow C. The residents of a territory to vote on whether to allow slavery. This doctrine was introduced as a way to settle the contentious issue of slavery's expansion into new territories, especially after the Mexican-American War when the United States acquired new lands. Popular sovereignty suggested that the people who settled in these territories had the right to decide whether slavery would be permitted.
Despite initially winning Lewis Cass the Democratic nomination due to its ambiguous timing, leaving hope for both pro- and anti-slavery factions, the application of popular sovereignty in places like Kansas led to significant violence and turmoil, such as the events known as "Bleeding Kansas." Ultimately, the doctrine did not prevent increased sectional conflict and highlighted the limitations of applying democratic principles to the highly divisive issue of slavery.