Final answer:
The rise in urban population by 1920 was primarily a result of urbanization, driven by industrialization that necessitated factory labor, drawing many individuals from rural to urban areas for employment.
Step-by-step explanation:
By 1920, more than half of the population of the United States lived in urban areas, with many newcomers leaving farms and small towns to build new lives in cities. This transformation was largely driven by urbanization processes. Job creation in urban areas was not due to agricultural advancements, technological stagnation, or rural development, but indeed due to urbanization. This was a result of industrialization that brought advances in technology and the need for factory labor, thus attracting both domestic migrants from rural areas and immigrants to the bustling cities for work opportunities.