B. It demonstrated Western Europeans' growing fears about the spread of communism.
Explanation:
Churchill described the Soviet threat as an "iron curtain." Clearly, he thought it was dangerous and pervasive. This was near the start of the Cold War tensions, so we can be sure that A and D are not correct. At this point, capitalism and communism are rallying their forces for defense or attack. And C doesn't make sense, because Churchill does not reference or imply nuclear power in any context in this quote.
