The correct answer is Containment.
The Truman Doctrine started to function in 1947, and established a new foreign policy approach that aimed to stop the geopolitical expansion of the so-called communist bloc, constituted by the Soviet Union and the Eastern countries under its influence. It did not involve direct military interventions of conflict with them, in turn it targeted other countries (third parties) that were threatened by the Soviets due to their proximity or to certain ideological linkages with them, and tried to help and protect them from the Soviet influence, aiming to turn them to be under the influence of the US instead.