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

EDITORIAL LEARNING AID

Why is this answer correct?

Saxony belonged to the Soviet occupation zone and was a state of the GDR until 1990; it was re-established in 1990. Bavaria and Baden-Württemberg lay in the American zone, Lower Saxony in the British zone. All three were part of the Federal Republic.

Saxony belonged to the GDR.

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