Following the america civil war northern cities saw an increase in population thanks in part to

C. the flight of former slaves from the south.

Explanation:

After the civil war, the white citizens were legally prohibited to own salves. The federal government wanted all states to acknowledge that black people were legal citizens of United States just like White people.

But, even after legally freed from slavery, most people still cannot accept the fact that they're equals with the former results. Black people constantly faced with discriminations , not treated fairly in the court of laws, not hired  by business institutions, etc.

This situations were much worse in southern states (which mostly affiliated with the confederates)  compared to the northern states (which mostly affiliated with Abraham Lincoln and the Union). As a result, black people started to move from southern states to the northern states, causing an increase in northern states' population.

a photo collage of women at work during world war 2