Question

what river is a famous american river that once was the western border of the united states?
mississippi river
ohio river
missouri river
kentucky river

Explanation:

To determine the river that was once the western border of the United States, we recall historical and geographical knowledge. The Mississippi River was a significant boundary in U.S. history, including serving as a western border at one point. The Ohio, Missouri, and Kentucky Rivers do not match this historical boundary role.

Answer:

A. Mississippi River