Which federal state is a city-state?

EDITORIAL LEARNING AID

Why is this answer correct?

Hamburg is a city-state: the state consists only of the city of Hamburg. Saxony, Bavaria and Thuringia, by contrast, are area states with many towns and municipalities.

City-state means: the state and the city are the same.

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