How many federal states joined the Federal Republic of Germany during reunification in 1990?

EDITORIAL LEARNING AID

Why is this answer correct?

In 1990 five states acceded: Mecklenburg-Western Pomerania, Brandenburg, Saxony, Saxony-Anhalt and Thuringia. There were no four, six or seven states at reunification; East Berlin was merged with the existing state of Berlin and was not a new federal state.

1990: five new states, plus East Berlin joining Berlin.

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