Which action did Jefferson take that significantly increased the size of the United States?

The action that significantly increased the size of the United States was the Louisiana Purchase. In 1803, President Thomas Jefferson negotiated this deal with France, which resulted in the acquisition of approximately 828,000 square miles of territory. This land, which extended west from the Mississippi River to the Rocky Mountains, effectively doubled the size of the nation at that time and opened up vast areas for exploration, settlement, and economic development.

By securing this territory, Jefferson aimed to promote an agrarian lifestyle, which he believed was key to the nation's future. The move was significant not only for its immediate impact on U.S. territory but also for its long-term implications for American expansionism and the concept of Manifest Destiny. This event laid the groundwork for the later westward expansion of the United States and enhanced its geopolitical presence in North America.

Other actions listed, while historically important, did not have the same immediate and transformative impact on the size and territory of the United States as the Louisiana Purchase.