Final answer:
The true statement about the southern economy before the Civil War is that the southern climate was excellent for growing cotton. The cotton gin's invention increased the demand for slave labor, and cotton became a crucial part of the national economy due to its significance as an export and as a supply for northern mills.
Step-by-step explanation:
The statement about the southern economy before the Civil War that is true is: the southern climate was ideal for the cultivation of cotton.
The invention of the cotton gin by Eli Whitney in 1793 made the processing of cotton much easier and more profitable, leading to a boom in cotton production which further entrenched the use of enslaved labor. The warm and humid climate of the southern states was indeed ideal for growing cotton, which became the dominant crop in the region.
The enslaved population grew not due to the Atlantic slave trade, which was officially ended in 1808, but because of an internal slave trade within the United States. Cotton was extremely important to the national economy as it was a major export, especially to Great Britain, and provided the raw material for the mills in the North.