Which Balkan state most threatened the unity of Austrian Hungarian empire?

The Balkan state that most threatened the unity of the Austro-Hungarian Empire was Serbia. Serbia had a long history of opposing Ottoman rule and sought to unite all South Slavic peoples into a single state, which would have included many territories within the Austro-Hungarian Empire. This ambition was seen as a threat to the territorial integrity and stability of the empire, as it could potentially lead to the loss of significant territories and the rise of a powerful rival in the region.