Answer:
C. Austria-Hungary had annexed Bosnia and Herzegovina.
Step-by-step explanation:
The reason that the relationship between Austria-Hungary and Serbia dissolved was that Austria-Hungary had annexed Bosnia and Herzegovina. This act by Austria-Hungary was seen as a direct challenge to Serbian interests in the region and created tension between the two countries. The annexation of Bosnia and Herzegovina by Austria-Hungary in 1908 was met with strong opposition from Serbia and other Balkan states, leading to increased hostility and ultimately contributing to the tensions that erupted into World War I.