Manifest destiny is the correct answer. This was the belief that the US had the divine right to take all of the land between the east coast and west coast.
These are the reason why the other questions are incorrect:
Monroe doctrine: This was a doctrine where the US would attempt to stop further European colonization into the Americas.
Isolationism: This was a policy where the US would not join any foreign wars, nor join any alliances.
Expansion: Though this is technically correct, the proper name for it (and the correct answer) would be Manifest Destiny.
Hope this helps!
