What doctrine promised U.S. support to countries resisting communism?

The Truman Doctrine is the correct choice because it was established in 1947 in response to the geopolitical tensions of the time, particularly the threat of communism in Europe and around the world. The Doctrine asserted that the United States would provide political, military, and economic assistance to countries that were resisting subjugation by armed minorities or outside pressures, primarily from the Soviet Union.

This policy aimed to contain the spread of communism and marked a significant shift in U.S. foreign policy, positioning the nation as a global leader committed to fighting communism wherever it emerged. It became the foundation for American involvement in various conflicts during the Cold War, such as in Greece and Turkey, and influenced U.S. actions in other regions.

Other doctrines and plans, while significant in their own contexts, do not specifically encompass the same promise of support against communism. The Monroe Doctrine focused on limiting European colonialism in the Americas, the Eisenhower Doctrine extended this principle to the Middle East, and the Marshall Plan was primarily an economic aid program to help rebuild European economies after World War II, rather than a direct military or political commitment to opposing communism.