Which agreement normalized diplomatic relations between Germany and the Soviet Union?

The agreement that normalized diplomatic relations between Germany and the Soviet Union is the Treaty of Rapallo. Signed in 1922, this treaty marked a significant moment as it established a basis for cooperation between the two countries after World War I, particularly in light of the diplomatic isolation both found themselves in after the war. The Treaty of Rapallo not only addressed the issue of war reparations that Germany owed as per the Treaty of Versailles but also laid the groundwork for various economic and military agreements between the two nations, allowing them to bypass the restrictions imposed by Western powers.

The other agreements mentioned do not serve the same purpose of normalizing relations between Germany and the Soviet Union. The Treaty of Versailles primarily dealt with the consequences of World War I for Germany, while the Kellogg-Briand Pact was an agreement to renounce war as a means of resolving disputes, and the Young Plan was a proposal for the reduction of Germany's reparations payments post-World War I. None of these treaties reflect the specific diplomatic normalization that the Treaty of Rapallo achieved.