How did the Federal Republic of Germany and the GDR become one state?

EDITORIAL LEARNING AID

Why is this answer correct?

On 3 October 1990 the five eastern states acceded to the Federal Republic of Germany; the legal basis was Article 23 of the Basic Law as it then stood. There was no occupation, and the western states did not accede to the GDR. The two occupation options and the reverse accession are therefore wrong.

1990: the eastern states accede to the Federal Republic.

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