c) James Monroe

James Monroe, who would later become the fifth U.S. President, was instrumental in the successful negotiation of the Louisiana Purchase as the Minister to France. His diplomatic skills and understanding of both American and European political landscapes were crucial in this landmark deal. Working alongside Robert R. Livingston, Monroe navigated the complex international dynamics of the Napoleonic era, securing a deal that was beneficial for the young American nation and significantly altering its future trajectory by drastically expanding its territory.