Final answer:
The distribution of ethnicities in the United States is characterized by concentrated ethnic clusters.
Step-by-step explanation:
The correct answer for the given question is C) Concentrated ethnic clusters.
Ethnic distribution in the United States is characterized by concentrated ethnic clusters, where certain ethnic groups dominate specific regions or areas. Examples of this can be seen in various ethnic regions such as German-American regions in Wisconsin, or areas with a high concentration of African Americans in the South.