Let's think about this question. If people moved into Northern cities like Chicago and New York, their populations are bound to grow a lot, right? I think we can safely say "A" and "B" are correct. This migration would also lead to the spreading of African American culture, so black poets WOULD become well known--C could be a result, too. But D, more blacks of farms than any earlier time in US history, doesn't make sense. The African Americans are moving AWAY from farms INTO CITIES, right? They'd be on farms a lot more in the past, when they were enslaved and working on plantations. So, D is not a result of the migration.
Answer: D