Answer:
The main cause of the Civil War was slavery, which is best represented by option D, "Slavery was legal in the South but illegal in the North." The issue of slavery was the most significant and divisive sectional issue in American history, and it was the underlying cause of the secession of the Southern states and the subsequent Civil War. The Southern states believed that they had the right to own slaves, while the Northern states opposed slavery on moral and economic grounds. The conflict over slavery ultimately led to the secession of the Southern states and the Civil War, which resulted in the abolition of slavery and the reunification of the United States.