The correct answer is C) urbanization.
The term that best describes what was happening between 1900 and 1920 was urbanization.
There were some important factors that influenced this urbanization process. First, the Industrial Revolution created many industries and factories in many large American cities, as was the case of Chicago or Nee York City.
People who lived in the rural regions of the South dedicated to stopping farming and moved to these large cities to work in the factories. They accepted low-paid jobs under unhealthy conditions, but that offered a better life than farming in the South.
Another factor was immigration. Thousands of immigrants from Europe and Asia arrived at these cities to offer their families a better way of living. Once in these cities, they had to live in poor and overcrowded spaces because these people were poor also and had no money to invest in better housing.