Answer:
Jim Crow era is the main causes for the Great Migration.
Step-by-step explanation:
During the Jim Crow era, the Southern states introduced a Racial segregation. This segregation separated the infrastructure that exist in the states. The better infrastructures tend to be allowed for white citizens only while the worse infrastructures were given for the Black citizens.
This resulted in a lot of suffering and missed opportunities for the Black citizens.
As a result, a large portion of black citizens decided to migrate to northern states. They heard that the north had a lot more job opportunities for them and the treatment that they'll have there wouldn't be as bad as they receive in the South. This period of mass migration is what we usually refer to as the Great migration.