Final answer:
On the eve of the Civil War, the Southern United States supplied approximately 90 percent of the world's cotton, dominating the global market and playing a significant role in international trade and political strategies.
Step-by-step explanation:
On the eve of the Civil War, the southern United States dominated the world's cotton supply. To understand the magnitude, we see that approximately 75 percent of the cotton produced in the United States was exported internationally, and during the 19th century, particularly between 1820 and 1860, about 80 percent of the global cotton supply was produced in the United States.
By 1860, the Southern states were responsible for providing a majority of the cotton Britain imported. In fact, by the time of the Civil War, the phrase 'Cotton is King' was used by South Carolina politician James Hammond to assert the dominance of the Southern states in cotton production. This near-monopoly was significant as the South attempted to leverage Britain's dependency on their cotton for diplomatic support during the secession. Consequently, the answer to the student's question would be the first option: 1) 90 percent of the world's cotton supply came from the southern United States.