The correct answer is the first option: the speech convinced Americans that the USSR was an enemy, rather than an ally, which led to the creation of the Truman Doctrine.
Winston Churchill's speech (gave in March 1946 in Missouri) produced a strong impact on Americans and president Truman. He referred to the USSR as an "iron curtain" to express how it was a big problem for all the rest of the countries because it had many military and economic allies, which made the USSR powerful as well as dangerous.
As a result, the Truman Doctrine was created by President Truman to show that the US wouldn't be an ally of the USSR. Instead, it would provide military, economic and political assistance to democratic nations who were in danger of any form of authoritarianism.