Step 1: The G-20 Summit is an annual meeting of leaders from the world's 20 major economies.
Step 2: The 12th edition of the summit was held in Hamburg, Germany, in July 2017.
Step 3: It focused on global trade, climate change, and international cooperation. \[ \boxed{\text{Hamburg}} \]
Since all four cities are in Germany, the distinguishing fact is which one actually hosted the summit, not general geography.
Records of the event confirm Hamburg as the host city of the 12th G-20 Summit.
So, the correct answer is Hamburg.