What does "Rechtsstaat" (a state under the rule of law) mean in Germany?

EDITORIAL LEARNING AID

Why is this answer correct?

In a state under the rule of law, the state itself is bound by statute and constitution (Art. 20(3) GG). Answer a is merely a play on words, b confuses "right" with "right-wing parties", and c describes popular sovereignty, not the state's binding to the law.

Rule of law: the state too must abide by its own laws.

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