Answer: D.
Step-by-step explanation:
The answer is D. "Southern states argued that the states themselves had created the Federal government and could leave the Union."
This is the correct answer by process of elimination, because A and B are antithetical to logic (why would they secede from the union if they believed that secession was immoral/illegal?).
C is incorrect because they did not believe the President had to be petitioned to leave the union, just that the states had to agree to leave.
Therefore, D is correct as it best matches the most common justification for secession.