The cotton gin fundamentally changed agriculture in the South by making cotton the dominant crop. Prior to its invention, cotton was labor-intensive to process, but the cotton gin allowed for the quick separation of cotton fibers from seeds, leading to a dramatic increase in cotton production. This shift not only made cotton the primary agricultural product of the South but also reinforced the use of slave labor on plantations to meet the growing demand for cotton, particularly in the wake of the Industrial Revolution and the rise of textile manufacturing. So the correct response is:
It made cotton the dominant crop.