The answer is: D.)The invention of modern power
The invention of modern power (such as electricity) make factories able to increase the productivity of their machines with relatively low price. Before electricity, factories have to rely on oil or coals. Both of them are more costly compared to electricity.
This make it possible for factories to increase the number of production without burning too much capital. Because of this, the invention of modern power become one of the most crucial factors that cause the growth of factory system in united states.