Answer :
"A. The United States built a series of defensive fortifications in the 1950s called the Iron Curtain" is incorrect. The Iron Curtain was built by the Soviet Union in Germany.
The correct answer is A.
The expression 'Iron Curtain' was methaporically used to refer to the confrontation that existed during the Cold War era between the Eastern bloc of communist countries leaded by the URSS, and the Western bloc of capitalist countries headed by the US.
In fact, the Iron Curtain splitted Europe in two halves, the communist East and the Western countries that were US allies.