Answer:
b. States' Rights
Step-by-step explanation:
The seven states in the Deep South based their right to secede on the theory of "States' Rights"
This is because the theory of State's Rights is based on the idea that States possess political powers which makes the national government an agent of State governments based on the United States Constitution.
Hence, these states believed that they have the right to secede because they joined the Union based on their own volition and rights, and they can use the same rights to secede.