Step 1: Read the question.
We must find which treaty led to the forming of the League of Nations.
Step 2: The background.
World War I caused huge loss of life. After it ended, world leaders wanted a way to stop such wars in the future.
Step 3: The peace treaty.
In 1919 the Treaty of Versailles was signed between Germany and the Allied powers to settle the war.
Step 4: The new body.
One important part of this treaty was the setting up of the League of Nations, the first big organisation meant to keep world peace.
Step 5: Rule out the others.
The Treaties of Paris, Vienna, and Berlin dealt with other times and issues, not the League of Nations.
Step 6: Conclude.
The League of Nations was formed by the Treaty of Versailles.
\[ \boxed{\text{Treaty of Versailles}} \]