The purpose of the Truman Doctrine of 1947 was to decrease the risk of nuclear war by placing atomic weaponry under the control of the United Nations A prevent communism from spreading further through military aggression B retake Eastern Europe from the Soviet Union C prevent the spread of independence movements in European colonies in Asia and Africa D create an atmosphere of trust in which negotiation with the Soviet Union would have the best chance for success

The right choice is:

To prevent communism from spreading further through military aggression.

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.

c. as a force against soviet invasion of afghanistan