The best answer is A.
Popular sovereignty was a doctrine under which the status of slavery could be determined by the settlers themselves. Although the doctrine won wide support as a means of avoiding conflict over the slavery issue, its meaning remained rather ambiguous since those who supported it disagreed as at what stage as territories developed should the decision be made.
A. Douglas, a main promoter of the doctrine, wanted the choice to be made at an early stage of settlement, while others felt that it should be made just before each territory achieved statehood. It was first proposed by vice president George Dallas in 1847 and popularized by Lewis Cass in 1848.