World war I started because of the assassination. It led to the destabilization of the nationalist to desire independence as they wanted to form independent multiple states. Thus, option C is correct.
Nationalists are the group of people that believes in the idea of nationalism that wants to support the best interest in the favor of their nation. They became a threat to Austria-Hungary.
Austria-Hungary was a country that had people of many cultures and nationalities and if the nationalists divided them according to their ethnicity then the original culture and togetherness of the country will perish.
Therefore, option C. Nationalists wanted different states.
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