What doctrine declared that European powers should not interfere in the affairs of the Western Hemisphere?

The Monroe Doctrine is the correct answer because it was a key U.S. policy announced in 1823 that asserted the Western Hemisphere was closed to further European colonization and interference. This doctrine articulated the idea that the United States would view any attempts by European nations to extend their political influence into the Americas as a direct threat to its own security and peace. It established a key principle of U.S. foreign policy, asserting the region's independence from European powers and laying the groundwork for the U.S. role in regional affairs moving forward.

The Truman Doctrine, on the other hand, was aimed at containing communism during the Cold War and does not pertain specifically to the non-interference of European countries in the Western Hemisphere. The Roosevelt Corollary expanded upon the Monroe Doctrine but focused primarily on justifying U.S. intervention in Latin America under certain circumstances, rather than establishing a blanket policy against European interference. The Marshall Plan was an American initiative to aid Western Europe economically after World War II, also unrelated to the topic of European non-interference in the Americas.