Answer:
D. Ethnic rivalries that erupted into violence
Step-by-step explanation:
Yugoslavia was a multi-ethnic state, with people groups like the Bosnians, Croats, and Serbians (to name a few) who did not get along due to the ethnic, religious, political, and cultural differences. After Yugoslavia dissolved, the ethnic borders for each ethnicity were not clear at all and the Catholic Croats, Muslim Bosnians, and Orthodoxy Serbians began one of the bloodiest wars in Europe since WW2.