Hello!
The correct answer is the fourth option - The Southern colonies' economy had growing labor demand.
In the 17th century, the South relied heavily on agriculture and farming. Slavery was seen as cheap labor, which farmers used to their advantage. Farming was extremely physically demanding and time consuming. Farmers relied on slaves to work and help maintain farms for very cheap.
Slavery was not depended on as heavily by the northern colonies due to rapidly growing industrial factories.
I hope this helps you! Feel free to comment or PM me if you have any additional questions! Have a great day!