What outside factor most directly contributed to the formation of enclaves in cities?

The correct answer is housing discrimination.

This is a type of discrimination in which someone is treated unfairly when trying to buy, rent, lease, sell or finance a home, based on characteristics, such as race, class, sex, religion, national origin, and familial status.

In the United States, housing discrimination began after the abolition of slavery as part of a federally sponsored law but has since been made illegal. However, it still remains a factor in the persistence of racial enclaves in American cities.

i would go with the last one