The correct answer is - C. labor brought about by large-scale farming.
The African slaves were brought to the English colonies for the purpose of them to work on the big farmlands. The African slaves were well known for their strength and endurance, and also they were used to living and working in places that are warm and humid. The big land owners in the English colonies were buying them because they were the cheapest labor force, but also the most effective labor force at the same time. As there were more and more farms, the demand for African slaves was increasing more and more.