Answer:
People and supplies from entering or leaving West Berlin.
Step-by-step explanation:
The USSR and the United States had been in a cold war which resulted in the Berlin Crisis of 1961. This event resulted in the construction of the Berlin Wall in 1961.
Before this construction, the USSR and America had been in constant attempts to prevent each other from accessing their 'areas'. These areas refer to the parts of Berlin, with the Eastern side under the Soviets and the Western side under the United States. The Soviets then blocked any land access to the West side of Berlin in order to prevent anyone from leaving or entering West Berlin. The preventive measure forced the United States and its allies to start the airlifting of food and other necessities which came to be known as "the Berlin Airlift".
Thus, the correct answer is the last/ fourth option.