Answer:
I believe it is A. Social safety nets
Explanation:
The Light Bulb, existed back then, but was very expensive to run, and needed a lot of power. This meant that regular consumers, could not use it. With the invention of the incandescent bulb, anyone in any household could flick a switch. This jumpstarted new inventions, which make the world what it is today.
Those were the "roaring twenties". It was a time of prosperity right before the great depression. The economy was booming.