Which factor most affected the balance of power and led the start of world war 1 ?

is Ethnic unrest in the Balkans and Ottoman Empire caused conflict between the alliances.

In 1912 and 1913, just before the assassination of Archduke Franz Ferdinand and the start of World War I in 1914, the Balkan Wars would leave a trail of disaster as four Balkan states defeated the Ottoman Empire. This war was caused because first of all the Ottoman Empire could not deal with the diverse ethnicity of the people it governed, much less with its rising nationalism, also because the Empire failed to govern and to reform itself to govern more efficiently; the Balkans felt they had to take the issue into their own hands, and finally the four Balkan states had formed the Balkan league and were confident together they could be free of the Turks. As this war took place, the alliances began to test each other: who was willing to take the chance to defend its allies in these wars? Everyone was saving their millitary for their own enemies. No one wanted to use them on another one's quarrel. Tension rose and this prepared the ground for the final straw.

Explanation:

d is the answer. leo should include the source.