C) to prevent East Germans from escaping to West Germany.
With Berlin being located in East Germany, the city provided a quick and easy opportunity for those under Soviet control to move to the democratically controlled portion of Germany.
The Soviet Union were dealing with people in East Germany and the eastern portion of Berlin moving into the democratically controlled West Berlin. The US, France, and Great Britain controlled the western portion of Germany and West Berlin. To prevent this movement, the Soviet Union constructed a wall around West Berlin overnight on August 13, 1961. The wall remained in place until November 9, 1989 when it was torn down.