Answer: Harry S. Truman
Step-by-step explanation:
Winston Churchill was the leader of Great Britain, so it is unlikely he ever would have given a speech to the US Congress.
Josef Stalin was the leader of the USSR and would not have given a speech before Congress, let alone to ask for assistence for Turkey and Greece (which the US was trying to prevent from being taken over by Communists)
Based on the job of the president, it makes the most sense that this speech would have been delivered by President Truman. The president is the supreme commander of the military, but must ask Congress for funding for the military. Also, this speech communicates what is known as "The Truman Doctrine", which is to provide aid to free countries to prevent them from becoming communist.