The response that best analyzes how the shift in population of the Three Forks area in the early 1800s impacted the economy is:
Indigenous groups from the Southeast were forcibly moved into the area, bringing with them new agricultural items to trade.
This response indicates a direct economic impact through the introduction of new agricultural products that could enhance trade and the economy in the region. The forced relocation of populations typically leads to changes in trade networks and the introduction of new goods, which would affect the local economy positively by diversifying what was available for trade.