Which federal state is a city-state?

EDITORIAL LEARNING AID

Why is this answer correct?

Berlin is a city-state: the city and the federal state are identical. Saarland, Brandenburg and Hesse are area states with many cities and municipalities; apart from Berlin, only Hamburg and Bremen are city-states.

City-states: Berlin, Hamburg, Bremen.

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