Southern agriculture was critical to the development of the Southern states, as it enabled them to use free and forced labor in order to see large profits. Lets take a look at each option:
A) The plantation system was efficient and profitable, leading to growth in Southern agriculture. (The plantation system was not super efficient, as relying on slave labor resulted in many issues)
B) Most South Carolinians could afford to run a large plantation, increasing production of agricultural goods. (Most people from South Carolina could not afford much, as only the top percent of the population was able to open and run plantations.)
C) The plantation system eliminated the need for slaves, allowing them to work on smaller farms. (NO! The plantation system CREATED the need for slaves)
D) The development of the plantation system allowed cotton to become a major export of the South. (YES! Cotton became a backbone of the Southern exports. Whenever we think of the south during and prior to the civil war, we think of cotton plantations, which were entirely dependent on slave labor.)
This means that the answer is D- The development of the plantation system allowed cotton to become a major export of the South.
I hope I've helped! :)