The correct answer is: It means that it reopened the debate over the border between free and slave states.
This interpretation aligns with the idea that popular sovereignty, which allowed new territories to decide for themselves whether to allow slavery, undermined the previously established boundaries set by the Missouri Compromise, which had aimed to maintain a clear division between free and slave states. By introducing the possibility of slavery in areas where it had previously been prohibited, it reopened contentious debates over the status of territories, thus challenging the effectiveness of the Missouri Compromise.