Final answer:
The decline in the prices of farm goods in the late-nineteenth-century United States was primarily caused by overproduction of agricultural products.
Step-by-step explanation:
The decline in the prices of farm goods in the late-nineteenth-century United States can be attributed to the overproduction of agricultural products. Due to the westward expansion of homestead farms and advancements in farm tools, crop yields increased significantly, leading to an excess of products in the marketplace. As a result, prices dropped. This overproduction was one of the largest challenges faced by American farmers at the time.