The correct answer is C) because an attack on one country could lead to that country's allies becoming involved in the conflict.
The system of alliances in Europe before the First World War was a threat to peace because an attack on one country could lead to that country's allies becoming involved in the conflict.
This system of alliances was created to form a more solid front with the union of different nations that tried to maintain or defend their economic and political interests in Europe. So if one of those nations had conflicts with its enemy, it would receive the support of its allies.
That is how France, Great Britain, Russia, and the United States formed teh Allied Powers.
On the other hand, Austria-Hungary, Germany, and Italy formed the Triple Alliance. Italy left prior to WWI. The Ottoman Empire and Bulgaria entered the alliance prior to World War I. They were known as the Central Powers.
