D. Tariffs.
Explanation:
Tariffs are basically taxes that are put onto foreign exports and imports, meaning if you buy overseas it will be more expensive than buying in your own country. This also meant that products being made in the United States looked less desirable to foreign countries because there was a tariff on them.
The South hated tariffs, as they had focused on agriculture and that meant most of the time they were selling overseas. They knew that if a high tariff was going to be put on their products, other countries may not want to buy them anymore. This also made it more expensive for them to buy overseas, which they hated. The North favored tariffs, as they were focused on industrialization, which meant the tariff forced people to buy from American businesses because it was cheaper.
South Carolina despised tariffs and even enacted the Ordinance of Nullification. This essentially said that tariffs were unconstitutional, that South Carolina thought of them as null and void, and that they weren't going to follow them anymore. They threatened to secede from the Union if the tariffs were not lowered, which eventually did cause the tariffs to be lowered.
