Final answer:
The free enterprise system greatly benefited the Southern United States prior to the Civil War through the cotton boom, making cotton cultivation the dominant economic activity.
Step-by-step explanation:
The free enterprise system in the United States before the Civil War notably benefitted the South through its cotton boom. This era saw the South producing the majority of the world's cotton supply, with large agricultural plantations supported by slave labor cultivating cotton for mills in the North and for export. This agricultural focus resulted in economic prosperity for the region and helped to fuel the market revolution transforming the United States. However, while there were some developments in railroad building and textile factories in the South, and a cattle boom much later, it was cotton that most profoundly benefited from the free enterprise system in the Southern states prior to the Civil War. Therefore, the correct answer to the question would be D. cotton boom.