During which crisis did the U.S. and Soviet Union face heightened tensions related to Berlin?

The correct answer is the Berlin Crisis, which refers to the series of events that heightened tensions between the U.S. and the Soviet Union regarding the status of Berlin in the late 1940s and early 1960s. This period was marked by significant confrontations, most notably the Berlin Blockade in 1948, when the Soviet Union attempted to restrict access to West Berlin in response to the Western Allies' efforts to unify Germany. The response from the U.S. and its allies included the Berlin Airlift, which was a massive operation to supply West Berliners with food and fuel, thereby countering the Soviet blockade. This crisis exemplified the ideological divide of the Cold War, with Berlin symbolizing the struggle between communism and democracy.

While the Cuban Missile Crisis also involved heightened tensions between the two superpowers, it was primarily centered on the establishment of Soviet missiles in Cuba rather than Berlin. The Iron Curtain Crisis is not a commonly recognized term in historical discourse. The Korean Conflict, while significant, pertained to hostilities on the Korean Peninsula rather than the specific issues surrounding Berlin. The Berlin Crisis is thus the most accurate choice, as it directly captures the conflict over Berlin's status and the associated geopolitical tensions.