Which present-day German federal state formerly belonged to the territory of the GDR?

EDITORIAL LEARNING AID

Why is this answer correct?

Brandenburg belonged to the Soviet occupation zone and was part of the GDR until 1990; the state was re-established in 1990. Bavaria and Hesse lay in the American zone, and the Saarland belonged to the French zone. All three were part of the Federal Republic.

Brandenburg belonged to the GDR.

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