Final answer:
The best description of the United States after the Louisiana Purchase is that it significantly increased its size, as the purchase doubled the nation's territory.
Step-by-step explanation:
The statement that BEST describes the United States as a result of the Louisiana Purchase is C. It increased its size significantly. The Louisiana Purchase effectively doubled the size of the United States at a cost of $15 million, a substantial bargain for the amount of territory acquired.
President Thomas Jefferson's administration overcame constitutional qualms to secure this vast expanse of land, to ensure that the Mississippi River and the port of New Orleans would be under American control, supporting the vision of an agrarian republic and facilitating westward expansion and agricultural commerce.