How often are there normally Bundestag elections in Germany?

EDITORIAL LEARNING AID

Why is this answer correct?

The Bundestag is normally elected every four years (Article 39(1) of the Basic Law); the electoral term can be shortened only in exceptional cases. Three, five or six years are not regular intervals between elections.

Bundestag elections: normally every four years.

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