Final answer:
Cotton became the major cash crop of the South by the 1850s, following the widespread adoption of Eli Whitney's cotton gin and the subsequent boom in cotton production.
Step-by-step explanation:
Cotton became the major cash crop of the South by the 1850s. The invention of the cotton gin by Eli Whitney in 1793 revolutionized cotton production by making it easier to separate seeds from the short staple cotton, which could be grown inland. This increased the efficiency and profitability of cotton production, leading to a significant increase in cotton output. By the mid-19th century, the United States produced about 68 percent of the world's cotton, and by 1850, 1.8 million enslaved people were producing cotton. By the 1850s, cotton had certainly cemented itself as the key cash crop of the southern economy, making option b) 1850s the correct answer to the student's question.