Hello. You did not provide the answer options, but one of the most significant events that show the failure of popular sovereignty was the sanction of the Kansas-Nebraska Act.
This act allowed Kansas settlers to decide, by vote, whether the state would be free or pro-slavery. The idea seemed good in the eyes of the settlers of KAnsas, but it proved to be a great disaster, since abolitionist settlers migrate in large numbers to Kansas to influence the vote, aiming against slavery. These settlers even provoked the murder of settlers who owned slaves, to frighten and manipulate the population and make them vote against slavery.