I believe the answer is urbanization, I'm not sure you should rely on my answer though.
The implementation of the assembly line by Henry Ford at his automobile plants was extremely important. It was important because it drastically reduced the amount of time it took to produce a Ford car. Since it took less time and effort to make the car, the price of Ford's dropped significantly. This drop in price made Ford cars more available to the American public, increasing the amount of people who owned cars in the US during the 1920's.
The development of the assembly line changed everyday life for Americans in multiple ways. First, traveling became much easier as cars were now available to more Americans. Along with this, the assembly line would be a method used by thousands of other companies all across the US. This made it so that workers would need to be able to complete one repeatable task throughout the entire day.
I think C. will be the right choice!
