Answer:
The right choice is:
To prevent communism from spreading further through military aggression.
Step-by-step explanation:
The Truman doctrine said that the United States would come to the aid of any country under a threat of internal forces (meaning a communist insurgency) or from an outside force (Soviet military aggression) to its domestic order. Washington would use all means available such as economic aid, military support or diplomatic initiatives. The doctrine was articulated by the Harry Truman administration to guide US policy of opposing communist expansion.