Answer:
D. nationalism
Step-by-step explanation:
Nationalism is the feeling of belonging to a specific culture, region, language and people (or, in some nationalist arguments, to a race). In the case of Serbia, since the 18th century the desire to unite the Serbian language countries was strong - Great Serbia, but the presence of the Austro Hungary Empire was a threat to this nationalist idea. When Franz Ferdinand attempted to incorporate Serbia to his rising empire, nationalists such as Gravilo Princip fought against.