What invention by Eli Whitney significantly impacted the Southern economy?

The cotton gin, invented by Eli Whitney in 1793, significantly transformed the Southern economy. This machine revolutionized the process of cotton production by efficiently separating cotton fibers from their seeds, which was previously a labor-intensive and time-consuming task. The cotton gin allowed for a dramatic increase in the amount of cotton that could be processed in a given time, leading to a boom in cotton as a cash crop.

As a result, the Southern states became increasingly dependent on cotton cultivation, which fueled the demand for slave labor and played a key role in the expansion of plantation agriculture. This shift not only enhanced economic prosperity in the South but also solidified its social structures and ultimately contributed to the complexities and tensions that would lead to the Civil War. Whitney's invention had a lasting impact, making cotton the dominant agricultural product in the region for decades.

Other options like the plow, steam engine, and reaper, while important agricultural and industrial innovations, did not have the same direct economic and social implications for the South's reliance on cotton production.