(A) An aggressive Germany was to blame for starting World War I.
(B) ** Many of the main combatants in World War I had selfish reasons for entering into the conflict.
(C) Alliances were the leading cause of World War I.
(D) The assassination of the Archduke Franz Ferdinand is the only reason World War I happened.
EXPLANATIONS BELOW
Concept note-1: -The immediate cause of World War I that made the aforementioned items come into play (alliances, imperialism, militarism, nationalism) was the assassination of Archduke Franz Ferdinand of Austria-Hungary. In June 1914, a Serbian-nationalist terrorist group called the Black Hand sent groups to assassinate the Archduke.
Concept note-2: -The assassination of Austrian Archduke: Archduke Franz Ferdinand of Austria was assassinated by the Black Hand organization. This was the spark that set off World War 1, which killed 9 million people.