The correct answer is option 1: Slavery. The American Civil War, which took place from 1861 to 1865, marked the end of slavery in the United States. The war was primarily fought between the Northern states, known as the Union, and the Southern states, known as the Confederacy. The main cause of the conflict was the disagreement over the institution of slavery and its expansion into new territories. The Union ultimately emerged victorious, leading to the abolition of slavery with the passing of the 13th Amendment to the United States Constitution in 1865. The end of the Civil War and the abolition of slavery were significant milestones in American history, paving the way for greater civil rights and freedoms for all citizens.