What doctrine established U.S. assistance to democratic nations facing threats?

The Truman Doctrine is the correct answer as it was a pivotal policy established in 1947 that committed the United States to providing support to countries that were resisting authoritarian threats, particularly from the Soviet Union and communism. This doctrine articulated the U.S. stance during the early stages of the Cold War, emphasizing the need to support free peoples who were fighting against oppression and the spread of communism. It laid the groundwork for American foreign policy for decades by declaring that the U.S. would assist any nation that was threatened by subjugation or outside intervention, which in those times predominantly referred to communist influence.

The other doctrines mentioned are significant in their own contexts but do not directly relate to the U.S. assistance to democratic nations facing threats. The Marshall Plan was focused on the economic recovery of Europe after World War II, the Monroe Doctrine primarily dealt with European colonialism in the Americas, and the Roosevelt Corollary expanded on the Monroe Doctrine by justifying intervention in Latin America. While they all represent very important aspects of American foreign policy, it is the Truman Doctrine that specifically addresses support for democratic nations in times of crisis.