Answer: Choice C.
They worried that Lincoln would try to end slavery in the United States.
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Step-by-step explanation:
The issue of slavery was debated and fought over for many years before the election of 1860. It was only until Lincoln became president that sparked the southern states to secede, which led to the Civil War. Proof of this is found in the many Declaration of Secession documents produced by each state that left the union. This is basically a document explaining why they left the United States to form the Confederate States of America (CSA) aka the Confederacy.
In modern times, some people mistakenly claim that the Civil War wasn't over slavery but rather states' rights. This is simply false. The documents I mentioned prove that slavery was the core issue. More proof is the various states having issues with the fugitive slave act, in that the northern states didn't really adhere to the law to the level of the southern states' liking. I guess you could argue that states' rights were involved, but specifically the south fought to have the right to own slaves. In short, it's all about getting the correct context. Expanding that context, simply look at the decades preceding the war and notice all of the tension involving whether a new state was a free state vs a slave state.