The correct answer is A. A political change in Europe since the 1980s was that East and West Germany peacefully reunited under a democratic government.
The term German reunification refers to the political and social changes that occurred during 1989 and 1990 in Germany, which ended in the accession of the former German Democratic Republic (GDR) under the jurisdiction of the Federal Republic of Germany (FRG). The process resulted in only one Germany.
The political and economic reforms promoted by the President of the Soviet Union, Mikhail Gorbachev, and the democratic transition in other countries of Eastern Europe had as an indirect consequence the migratory exodus towards the FRG through Czechoslovakia and Hungary, and the subsequent social instability in the GDR. These events forced the resignation of the Politburo of the Unified Socialist Party of Germany, which was later banned. It also encouraged the opening of the borders between the Germanies, which triggered the fall of the Berlin Wall on November 9, 1989.
After a series of agreements, summits and official visits, and the recognition of the German-Polish border on the Oder-Neisse line, the governments of the United States, the United Kingdom, France and the Soviet Union signed the Treaty Two plus Four, which ruled in favor of reunification. It also validated the Unification Treaty, a legal instrument that authorized the adhesion of the GDR in the FRG according to the Basic Law of Germany on October 3, 1990. Since 1991, that date is celebrated on the Day of German Unity.