C. Prussia -- that's the one that doesn't belong in the list
After World War I, the empire of Austria-Hungary was divided into these four nations: Austria, Hungary, Czechoslovakia, and Yugoslavia.
The Austro-Hungarian Empire, also known as the Dual Monarchy, had been in place since 1867, when Hungary's insistence on greater self-governance within the Austrian Empire had led to a revised constitution where Austria and Hungary each had their own ruling monarch, but the two remained joined within the overall empire. Other national and ethnic groups had been under that empire as well, and the breakup of the empire in 1918 resulted in separate states for some, such as the Czechs and Slavs. However, even the creation of a Yugoslav state for all Slavs didn't account for all the ethnic diversity within that group. In the early 1990s, conflicts within Yugoslavia caused that nation to break up into a number of smaller states, including Croatia, the Republic of Bosnia & Herzegovina, Slovenia, and more.
