The correct answer is C. The United States and Great Britain launched the Berlin Airlift in response to the Berlin Blockade, by which the Soviet Union cut off Berlin's access to medical supplies and treatment.
The Berlin Blockade was the closing of the borders shared by the United Kingdom and the United States with the Soviet Union in occupied German territory. It was developed between June 24, 1948 and May 12, 1949.
It was imposed by the Soviet Union, and affected mainly the western area of Berlin, then controlled by the forces of the United States, the United Kingdom and France. It was applied in response to the monetary reform imposed by these countries. The first two decided to supply the western part of the city through the three air corridors that were legally required to allow the USSR through its territory. The blockade, which lasted eleven months, was ended by the Soviet Union on May 12, 1949.