The correct answer is A) to defend against a Communist attack.
In the context, the meaning of "to preserve Western Civilization" is to defend against a Communist attack.
During the tense years of the Cold War, the Soviet Union and the United States were in permanent conflict of interest about the economy and political issues in the world. The Soviet Union wanted to spread Communism around the World. They controlled or influenced countries such as Hungary, Bulgaria, Romania, Poland or Cuba, meanwhile, the United States tried by every mean to prevent or stop the spread of Communism.