The correct answer is A. The Dred Scott decision
Step-by-step explanation:
One of the main factors that caused the Civil War in the U.S. from 1861 to 1865 was the tension between the North and South states triggered by slavery because this was a common practiced in the South but was opposed by people in the Northern states who considered it unethical and immoral.
This tension increased with the Dred Scott decision in 1857 because in this case, the Supreme Court decided black people were not citizens, and therefore did not have the right to be free an were not protected by the Constitution. This made abolitionists in the North even more angry and willing to end with slavery.