The correct answer is the "Louisiana Purchase." The Louisiana Purchase refers to the acquisition of a vast territory by the United States from France in 1803. To find this answer, one could have used a search engine or consulted a history textbook to get information about the purchase of land between the Mississippi River and the Rocky Mountains. The search query could have been something like "purchase of land between Mississippi River and Rocky Mountains" or "territorial acquisition involving the Mississippi River and Rocky Mountains." The search results or textbook would have provided the answer as the "Louisiana Purchase."
