The correct response is: slavery and plantations.
During the period of slavery in the United States, African Americans were primarily settled on plantations in the southern regions, including the Mississippi Alluvial Plain region. Slavery was heavily dependent on labor-intensive agriculture, such as cotton production, which was prevalent in the Mississippi Alluvial Plain. On the other hand, the Ozark Mountains region, located further north, had a different economic structure, with less reliance on slave labor and plantations. Instead, industries such as mining and logging were more prominent in that area. Therefore, the settlement of African Americans was more prevalent in the Mississippi Alluvial Plain region due to its compatibility with the plantation system.