Final answer:
Augsburg is the German city known as the Mother of Diets because it is the birthplace of the Augsburg Confession, a key document of the Protestant Reformation.
Step-by-step explanation:
The German city known as the Mother of Diets is Augsburg. It is called so because it is the birthplace of the Augsburg Confession, a foundational document of the Protestant Reformation that outlined the principles of the Lutheran faith. The Augsburg Confession was presented to the Holy Roman Emperor in Augsburg in 1530.