The balkans were called the "powder keg of europe" because

A powder keg is literally a barrel of gunpowder, but it can also mean a dangerous or volatile situation. The Blakans were called the Powder keg of Europe because they caused a number of different conflicts that could bring the whole continent into war. For example, WWI started in the Balkans, after the assassination of Archduke Franz Ferdinand of Austria, in Sarajevo, Bosnia.

   Many conflicts have been set on fire in the Balkans and extended further

Explanation:  The Balkans were called the "Powder Keg of Europe" because many conflicts were initiated and literally exploded in the Balkans and later spread to the whole continent and even to the world, if not conflict, at least the impact of that conflict was felt in most of the then known world. Examples of this are, the assassination in Sarajevo 1914, that led to the First World War, then the First Serbian Uprising against the Ottoman Empire, which led to the awakening of national consciousness among many ethnic groups in the then empires, and later to the collapse of the Ottoman Empire . However, all these reasons should also be considered in the context of multi-layered influences and interests that have been intertwined for centuries.

These areas for centuries have been the zones of interest of both the Catholic Church and the Byzantine Empire, and many great powers have looked to achieve their political and religious influence in this area, thus securing a kind of defensive line towards the "other". Thus, many influences in the form of a lot of different political, religious, traditionalist, modernist beliefs, made up a suitable ground for sparking when it was of some interest, and so it was always easy to light up and explode.

d is the correct answer

~courtney