The correct answer is James K Polk.
Manifest destiny was the idea that it was a foregone conclusion that America would expand their territory/influence from the east coast to the west coast of the North American continent. This concept was influenced by presidents like James K Polk.
Polk was a huge advocate of manifest destiny, as he even threatened a war against Great Britain for territory in the modern day American Northwest. His mantra of "54'40" or fight revealed his dedication to expanding America's borders.
