As much as C. has a partiality towards the cause of The Great War, it's most likely answer B.
The Germans declared war on the Russian Empire, guaranteed the independence of Serbia, because of their alliance backing Austria, and Serbia which the Austrians declared war on as a result of the Serbian citizen who shot the heir to the throne.
The rest of Europe's involvement was a chain reaction of countries coming to the aid of another. France with defending Russia, Britain defending Belgium, etc.
