Which of the following most directly contributed to the developments described in the excerpt?

D. Improvements in production techniques

Explanation:

The 1920s saw an improvement in production techniques that led to the mass production of certain items, chief amongst them the motor vehicle.

People saw the opportunities that mass production could bring and applied it to other products such as radios and the result was a mass production that led to a mass consumption of radio because everybody wanted to own a radio and be connected to the rest of the America and the World.