Answer :
The main reason was that southern plantations needed a lot of cheap labour - and often labour that was very hard and free workers would demand good pay and less working time in the harsh conditions.
The correct answer is B) southern plantation owners needed more labor at lower costs.
A reason why slavery developed in the American colonies was that southern plantation owners needed more labor at lower costs.
Wealthy southern landowners owned large plantations that needed extra hands to do the hard work. That is why they favored slavery. Indeed the southern economy depended so much on slavery to produce the right cash crops that could be traded and exported to Europe. Plantation owners bought African slaves and put them to work in the farm fields at almost no cost.