The correct answer is D. Warsaw.
The Warsaw Treaty Organization (Warsaw Pact) was a mutual defense treaty between the Soviet Union and seven Eastern European countries: Bulgaria, Czechoslovakia, Hungary, Poland, Romania, and East Germany. It was established in 1955 in response to the formation of NATO. The Warsaw Pact’s headquarters were located in Warsaw, Poland.
The Warsaw Pact was dissolved in 1991, following the collapse of the Soviet Union.
Option A is incorrect because Berlin is the capital of Germany. Germany was not a member of the Warsaw Pact.
Option B is incorrect because Prague is the capital of the Czech Republic. The Czech Republic was not a member of the Warsaw Pact.
Option C is incorrect because Moscow is the capital of Russia. Russia was a member of the Warsaw Pact, but the Warsaw Pact’s headquarters were located in Warsaw, Poland.