Answer:
A.
Slavery was the main cause of the Civil War.
Explanation:
i would also agree with you because the other answer choices are weak and dont go along with the passage.
"These slaves constituted a peculiar and powerful interest. All knew that this interest was, somehow, the cause of the war. To strengthen, perpetuate, and extend this interest was the object for which the insurgents would rend the Union, even by war;" this section basically hands you this assumption.