Answer: A) By Balkan calls for independence, which contributed to instability in the region.
Context/details:
The Ottoman Empire had expanded across sections of the Middle East and into Europe during the 16th century -- including control of Greek and Balkan territories. Suleiman the Magnificent, sultan from 1520 to 1566, was responsible for much of the territorial expansion of the Ottoman Empire in that era.
By the beginning of the 20th century, though, the Ottoman Empire was known as "the sick man of Europe," and other European powers were waiting to carve up its territory after it fell. One region where the Ottomans had been losing their grip was in Greece and in the Balkan region north of Greece. Bulgaria, Greece, Montenegro and Serbia all had achieved independence from the Ottomans by the early 20th century -- but other neighboring powers wanted influence in the region also. The Balkans remained unstable, and political observers at the time worried that it would be a problem area where war could erupt that would pull other nations into the conflict. That's exactly what did happen.
When Austrian Archduke Franz Ferdinand was assassinated in Sarajevo in 1914 by Serbian nationalists, the reactions by the Austrian and Serbian governments kicked into gear various alliances that had been made. Russia stood ready to protect Serbia. Germany sided with Austria over against Russia. The Ottoman Empire joined with Germany. Before long the whole affair plunged Europe into war, pulling the other nations into the conflict in support of prewar alliances they had made.