What role did nationalism play in the outbreak of world war i?
a. nationalism convinced the people of europe that their own nation could take on any military threat.
b. nationalism led to a new era of peace negotiations.
c. nationalism caused countries to stop the alliance system.
d. nationalism caused a reduction in trade.

The definition of nationalism states that it is the strong belief that the interests of a nation is of higher regard than any foreign influenced interests. Therefore the answer to your question would most likely be A, although all answers above are directly/indirectly influenced by nationalism. During the World War the governments of the Western European nations required manpower and morale in order to fight against the German-Austrian army, and nationalism most likely sparked patriotism for their homeland. 

The last sentence of the gettysberg address