Who was the primary advocate of the United States' policy to prevent Soviet expansion during the Cold War?

Explanation

George F. Kennan, an American diplomat, is credited with formulating the policy of containment aimed at limiting Soviet influence during the Cold War. He detailed this approach in his 1946 "Long Telegram" and the 1947 "X Article." While George C. Marshall is associated with the Marshall Plan and Cordell Hull was Secretary of State in World War II, neither played a direct role in advocating containment. Therefore, the option "None of the above" is incorrect.