Final answer:
The failure of popular sovereignty prior to the Civil War can be attributed to the young age of the United States and the lack of consensus among state populations on the issue of slavery.
Step-by-step explanation:
The failure of popular sovereignty prior to the outbreak of the American Civil War can be attributed to various factors, but the statement that BEST explains this failure is option B: Even by 1850, the United States was still a young nation, and individual states were unaccustomed to exercising popular sovereignty. State populations were in many cases equally divided on issues and would not settle on the majority choices, particularly relative to slavery.
The idea of popular sovereignty, which allowed residents of a territory to decide for themselves whether to permit slavery or not, led to conflicts and divisions among states. In many cases, state populations were evenly split on the issue of slavery, making it difficult to reach a majority decision. This lack of consensus and the failure to settle on the majority choices, especially regarding slavery, contributed to the failure of popular sovereignty.
Furthermore, the federal government of the United States was not set up to allow the states to do whatever they wished, which refutes option A. Additionally, the statement in option D, that local populations were preoccupied and indifferent to the federal government, does not fully explain the failure of popular sovereignty as it overlooks the significant divisions and conflicts that arose on the issue of slavery.