Final answer:
The Kansas-Nebraska Act effectively ended the Missouri Compromise by allowing the territories of Kansas and Nebraska to decide the issue of slavery for themselves, contrary to the restrictions established by the Missouri Compromise. Therefore, the correct option is A.
Step-by-step explanation:
One of the more controversial effects of the Kansas-Nebraska Act was that it effectively ended the Missouri Compromise. The Kansas-Nebraska Act, established in 1854, proposed by Stephen Douglas, applied the notion of popular sovereignty to the territories of Kansas and Nebraska, allowing them to decide for themselves whether to allow slavery. This directly contravened the Missouri Compromise, which had banned slavery above the 36° 30' line within the Louisiana Purchase lands. Therefore, the answer to the student's question is that the Kansas-Nebraska Act effectively ended the Missouri Compromise.