What does the abbreviation CSU stand for in Germany?

EDITORIAL LEARNING AID

Why is this answer correct?

CSU stands for Christian Social Union; it is the sister party of the CDU and stands for election only in Bavaria. The variants a to c are invented and are not party names.

CSU means Christian Social Union and exists only in Bavaria.

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