C. A better transportation system to move more products.
Explanation:
The South had many reasons to expand for the West: they wanted to raise cattle, taste new adventure, there were mining opportunities, etc. However, their main reason was because the railroads not only gave them access to supplies only accessible from railway travel, but they could also ride them and transport things on them themselves. Railroad travel was just starting to come to the country, and the South wanted to experience it just as much as anyone else.
