Final answer:
The Dust Bowl most severely affected farmers in the Great Plains states during the 1930s, leading to the displacement of around 800,000 people. Hence, the correct answer is option D.
Step-by-step explanation:
The "Dust Bowl" had the greatest impact on d) farmers in the Great Plains states. This environmental disaster occurred during the 1930s and coincided with the Great Depression, exacerbating the economic issues of the time. The Great Plains, especially parts of Oklahoma, Kansas, Texas, and surrounding states, were most affected by severe drought and dust storms that rendered the farmland unproductive.
As a result, a significant number of farmers and those dependent on the agriculture industry, approximately 800,000 individuals, were displaced. While many of these affected individuals sought work in California, they frequently encountered hostility and were pejoratively labeled as "Okies," which intensified regional and racial tensions.