Which reference source would best help answer the question: "Which states were formed from the land west of the Mississippi River?"

The best reference source for answering the question about which states were formed from the land west of the Mississippi River is an atlas. An atlas provides detailed maps that highlight geographic features, including the boundaries of states. It often includes historical context or timelines that could illustrate how the states west of the Mississippi River were established over time.

Specifically, an atlas would allow you to visually identify the states that were created from the territories acquired through various means, such as the Louisiana Purchase, which significantly expanded U.S. territory. By examining the maps in an atlas, you would easily be able to see which areas were part of this land acquisition and how state lines were drawn.

While an encyclopedia might provide broader context about the historical events associated with the formation of these states, it would not be as effective in directly answering the geographic aspect of the question. A dictionary would not provide relevant information at all, as it focuses primarily on word definitions. Although a map could also be useful in showing state boundaries, an atlas typically offers more comprehensive information and context specifically related to states and geographic regions.