Question

How did nationalism play a role in sparking World War I?. . A. Serbia gained independence from the Ottoman Empire.. . B. Serbians wanted to unite with Serbians in Austria-Hungary.. . C. Belgians in Belgium wanted to unite with Belgians in Germany.. . D. Belgians held off German troops for two weeks.

Answer 1

Hello!
The nationalism was one of the great reasons for the war.  The answer is b), because the serbians want the "freedom" to their people, unifying their ethnic group in the region.
Hugs!

Answer 2

I believe the answer is; Serbians wanted to unite with Serbians in Austria-Hungary

Because of this view, Austrian Archduke Franz Ferdinand and his wife was assassinated by a member of Yong Serbian nationalist. This caused an uproar within Austria which lead to the declaration of war made by Austria-Hungary toward Serbia that started the world war I.