Final answer:
The Louisiana Purchase, made in 1803, was the event that doubled the size of the United States, marking a significant milestone in the nation's expansion. The correct option is C.
Step-by-step explanation:
The event that doubled the size of the US was the Louisiana Purchase. This historic acquisition took place in 1803 when the United States, under the presidency of Thomas Jefferson, bought approximately 828,000 square miles of land from France for $15 million.
This territory extended from the Mississippi River to the Rocky Mountains and more than doubled the territorial size of the United States, paving the way for westward expansion and playing a significant role in shaping the nation's history.