Answer:
A.
Step-by-step explanation:
In reality, I bet all the answers are at least partly correct.
But the reason (or excuse) that people often gave was that the voters (no black people of course!) of each state should decide if they wanted to allow slavery.
They called it 'Popular Sovereignty' and it was a twisted argument that we could use a democratic process (voting) to decide that a certain class of people would get no democracy at all, or even be recognized as having ANY rights, like the right to not be owned and murdered.