Final answer:
Thomas Jefferson was the president during the Louisiana Purchase in 1803, which doubled the size of the United States and was a key event in American history.
Step-by-step explanation:
The president who was in office during the Louisiana Purchase was Thomas Jefferson. He viewed the control of New Orleans and the Mississippi River as crucial to the United States, both for westward expansion and its agrarian economy. Despite his initial misgivings about the constitutional authority to make the purchase, Jefferson facilitated the acquisition of the Louisiana Territory from France in 1803, which notably doubled the size of the United States. The purchase was signed by Jefferson's representatives, Robert Livingston and James Monroe, and cost the U.S. $15 million, a remarkable bargain of the time. As a result, Jefferson's vision of an agrarian America spread across vast and farmable lands took a significant leap forward.