Which of the following has been mentioned by historians as a cause of world war i?

a. nationalism in the balkans
b. militarism of african nations
c. fighting between india and pakistan
d. an unprovoked attack on the u. s.

Option A, nationalism in the Balkans

Explanation:

Eastern Europe cluster of nations lying between the Austro-Hungarian and Ottoman empires was known as Balkans. Due to its strategic location, all the Europeans powers focused on Balkans’ event. Also,  the Balkans were  politically active as they had both national ethnicity and tension with in its national territories.  

In 1912, several Balkan nations  became part of military alliances as influenced by the Russians which later on led to the  formation of the Balkan League. This Balkan league waged several wars against the Ottomans in order to remove them from eastern Europe. After fighting for eight months, Balkan won this battle but they were disrupted and thus Serbian nationalist groups and led to World War I

Hence, option A is correct