Thuringia
The Thuringian Realm existed until 531 and later, the Landgraviate of Thuringia was the largest state in the region, persisting between 1131 and 1247. Afterwards there was no state named Thuringia, nevertheless the term commonly described the region between the Harz mountains in the north, the Weiße Elster river in the east, the Franconian Forest in the south and the Werra river in the west. After the Treaty of Leipzig, Thuringia had its own dynasty again, the Ernestine Wettins. Their various lands formed the Free State of Thuringia, founded in 1920, together with some other small principalities. The Prussian territories around Erfurt, Mühlhausen and Nordhausen joined Thuringia in 1945.

Q: Which Prussian territories joined Thuringia in 1945?
Prussian territories around Erfurt, Mühlhausen and Nordhausen