What Was the Main Function of the United States in Terms

What was the main function of the United States in terms of foreign affairs under the Truman Doctrine?

A) to create a domino effect by which communist governments would collapse and be replaced by democratic ones
B) to establish a network of allied nations in Europe to oppose the territorial expansion of the Soviet Union
C) to prevent the spread of communism and Soviet influence through financial and material incentives
D) to keep the U.S. out of foreign conflicts outside the Western Hemisphere and focus instead on domestic development