Southerners justified secession with the theory of

Constitutional rights.

Federal rights.

the Union's errors

State's rights.

I think the answer is state's rights. Please correct me if i am wrong.

I agree.

You are correct! The theory of "States' rights" was a key justification used by Southerners to support their secession from the Union. This belief argued that since the United States was formed through a compact between individual states, each state had the right to govern itself and could choose to leave the Union if it believed its rights were being violated.

To arrive at this answer, you can analyze historical documents, speeches, and justifications made by Southern leaders during the time of secession. By examining these primary sources, you can identify the prevailing argument of "States' rights" being used to support the secession. It is important to consider the context and historical events surrounding the Civil War to understand the motivations behind this justification.