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

Assuming that this is referring to the same list of options that was posted before with this question, the correct response would be "immigration," since people from the same foreign nation tended to live closer together. 

Opposition to the nra came to a head in 1935, when the conservative supreme court declared the agency unconstitutional, claiming the nra gave the executive branch regulatory powers that belonged exclusively to congress. this decision began a series of court decisions that overturned key elements of the new deal.