What is a German city-state?

EDITORIAL LEARNING AID

Why is this answer correct?

Along with Hamburg and Berlin, Bremen is one of Germany's three city-states: the city and the state are identical. Munich and Frankfurt are cities in Bavaria and Hesse respectively, and Erfurt is the state capital of Thuringia.

Only three city-states: Berlin, Bremen and Hamburg.

This translation and explanation are editorial learning aids. The official German wording and answer key govern the examination.