What outside factor most directly contributed to the formation of enclaves in cities?
a: a common culture among american citizens
b: restricted access to education
c: housing discrimination
d: the availability of jobs

I believe the answer is: C. Housing discrimination

Enclave refers to a part of town which mostly populated by the people from one certain ethnicity/cultural background. 
When housing discrimination exist, only people with a certain race or ethnicity are allowed to live on a particular neighborhood. This forced the people from other ethnicities to seek housing elsewhere which lead to the creation of enclaves.

Housing discrimination was the outside factor that most directly contributed to the formation of enclaves in cities.