After the civil war, urban populations increased. This growth was likely due to:

The increase in urban populations following the Civil War can be closely linked to the Industrial Revolution. This period brought about significant changes in manufacturing and labor practices, leading to the establishment of factories and industrial jobs in cities. People migrated from rural areas seeking employment in these new industrial sectors, resulting in a rapid growth of urban populations.

As industries flourished, cities became centers of economic activity, attracting not only former farmers but also immigrants who were drawn by the promise of jobs and a better life. This shift marked a significant transition in American society from agrarian to industrial, fundamentally altering the demographic and economic landscapes of the country.

The other options, while they may contribute in varying degrees to urban growth, do not comprehensively explain the exponential growth in urban areas during that time frame as effectively as the Industrial Revolution does.