Final answer:
The significant change that occurred in the early 20th century was the shift from a primarily rural population to one where the majority of people lived in urban areas, as a result of industrialization and urbanization.
Step-by-step explanation:
Among the changes that took place in the first two decades of the 20th century, one significant transformation was that the majority of the U.S. population began living in urban areas rather than rural areas. Industrialization and urbanization were highly interconnected forces that dramatically changed the American societal landscape. The rise of industrial jobs in urban centers attracted a large number of migrants; as a result, cities expanded rapidly both in population and economic influence.
By the 1920 census, it was evident that a majority of the population resided in urban areas, signifying that urbanization had become a defining feature of American life. This shift from a primarily agrarian society to one centered on industrial and urban workplaces marked a fundamental change in where and how Americans lived and worked. Therefore, the correct answer to the student's question is option c: The majority of the U.S. population lived in urban areas instead of rural areas.