Which agricultural tool was improved by Eli Whitney's invention?

Eli Whitney's invention significantly improved the cotton gin, a tool originally used for separating cotton fibers from their seeds. Before the cotton gin, this process was labor-intensive and time-consuming, making cotton production a less feasible agricultural option. Whitney's design dramatically increased the efficiency of this separation process, allowing for a much cleaner and quicker extraction of cotton fibers. This innovation not only boosted cotton production levels but also transformed the agricultural landscape of the Southern United States, spurring economic growth and increasing the demand for slave labor in cotton plantations.

While the other agricultural tools mentioned—seed drill, moldboard plow, and thresher—played important roles in farming practices, they were not directly impacted by Whitney's invention. The seed drill, for instance, focuses on planting seeds effectively into the soil, the moldboard plow assists in turning the soil, and the thresher is used for separating grain from husks. These tools were developed independently of Whitney’s contributions to the cotton industry.