Hey there! I'm happy to help!
Let's review the four answer options we have about the term used by Churchill in reference to the Soviet Union's rule over the Eastern European nations.
A. The Marshall Plan
B. The Iron Curtain
C. The Warsaw Pact
D. Nato
Answer A is incorrect. The Marshall Plan was a plan from the U.S. to help Europe recover from WWII.
Answer B is correct. The Iron Curtain was an unofficial boundary between the Communist Eastern and Non-Communist Western European countries. It was not an official term so it makes sense that it would be something touted by Churchill.
Answer C is incorrect because the Warsaw Pact was an alliance between Eastern European countries, and the name of the pact wouldn't be a specific term Churchill uses because everyone would due to it being official.
Answer D is wrong because NATO is the North Atlantic Treaty Organization, which is an alliance between North American and European countries.
Therefore, the answer is B. Iron Curtain.
Have a wonderful day! :D