ANSWER
On March 15th, 1991 the "Treaty on the Final Settlement With Respect to Germany" formally ended the rights of the four allied countries to Germany.
At the beginning of 1990, Germany was still two separate countries, the Federal Republic of Germany (West Germany) and the German Democratic Republic (East Germany), but they would not stay that way for long. The borders between the two had been opened the previous November, the Berlin Wall had been reduced to rubble, and citizens on both sides were calling for a united Germany.
Representatives from these two German nations, along with the four former allies from WWII, agreed to a series of talks aimed at bringing about German reunification. These talks, cleverly titled the "Two-Plus-Four" talks, produced the "Treaty on the Final Settlement With Respect to Germany", often simply referred to as the "Two-Plus-Four" treaty.
The treaty was signed in Moscow on September 12th, 1990, clearing the way for German reunification, which occurred almost immediately after, on October 3rd of the same year. However, the treaty did not officially take effect until it was ratified by all parties, and the "instruments of ratification" were delivered to Germany. The Soviet Union, not surprisingly, was the last to ratify the treaty, which it did on March 4th, 1991. The process was then completed when the "instruments of ratification" were deposited in Bonn, Germany on March 15th, 1991.
WHO GOT IT RIGHT: Robert Walker, Bob Milligan, and Steve Shuba.
