Answer choice (B) most strongly supports the authors' analysis of the Great Migration because it is the answer that most strengthens their hypothesis (which is mentioned in lines 23 to 25), where they say, "We propose that once started, migration develops momentum over time as current migration reduces the difficulty and cost of future migration."
The authors provide support for their hypothesis throughout the passage, most notably in lines 41 through 57. These lines all suggest that many of the migrants who moved to the North came with migrants that had previously lived in their geographic region in the South. Answer choice (B) would provide support for this, thereby strengthening the authors' analysis.
Answer choice (D) would not strengthen the authors' analysis; in fact, it would weaken it, as it lends credence to an alternative hypothesis for the cause of the Great Migration. If (D) were true, it would suggest that the increased efforts of the recruiters, not the decreased cost and momentum created by other migrants, is what accounts for the Great Migration. It does not support what the authors are suggesting in that the first migrants helped make subsequent migrations for members of their communities easier, leading to increased migration.