Wann kam das Burgenland zu Österreich?

Why this is the correct answer

After World War I and the dissolution of the Austro-Hungarian Empire, the Treaty of Saint-Germain-en-Laye assigned the predominantly German-speaking western counties of former Hungarian territory to Austria. Following negotiations and a plebiscite in which Sopron chose to remain with Hungary, the remainder of the territory formally acceded to Austria on 26 November 1921. This made Burgenland Austria's youngest Bundesland and the only one that had previously been part of the Kingdom of Hungary.