The correct answer is D) perfection of the assembly line process of production.
The assembly line was used by Henry Ford, especially during the 1920's, to make his famous Ford Model T's. Thanks to the assembly line, these cars were produced faster and more cost effectively than ever before. This drastically reduced the price of the cars and reduced the amount of time that workers needed to work in order to make the car. These all had positive effects on the American economy.