Final answer:
Thomas Jefferson and US negotiators were concerned that the Constitution did not explicitly authorize the acquisition of new land, which made them question their right to purchase the Louisiana Territory. Nonetheless, the potential benefits overruled these concerns, and the Louisiana Purchase was completed in 1803, doubling the size of the nation.
Step-by-step explanation:
The question asks why US negotiators and Thomas Jefferson thought they might not be able to buy the Louisiana Territory. While the options given, such as the land being too expensive or worries about the Native American population, are not without merit, the historical documents suggest that Jefferson's concerns were primarily constitutional.
The Constitution did not explicitly give the President the power to acquire new land, and since Jefferson and others adhered strictly to the Constitution, this lack of authority posed a significant concern. However, the desire for westward expansion and the opportunity to purchase the land at a bargain price from Napoleon, who was motivated to sell, ultimately outweighed these constitutional concerns. Livingston and Monroe completed the Louisiana Purchase for $15 million, which doubled the size of the United States.