The correct answer to this open question is the following.
The growth of population in cities during the late 1800s was known as urbanization.
The Industrial Revolution changed the lives of people in America. New factories in larger cities demanded workers to operate the machines and create many jobs. People from rural places decided to emigrate to these larger cities in order to get a job and a better living condition for their families. So cities started to grow. This phenomenon was called urbanization.