To investigate the reason the assassination of Archduke Franz Ferdinand sparked World War I, the most relevant option would be:
the collapse of the alliance system
The assassination itself was the immediate catalyst, but the underlying tensions in Europe were largely due to the complex alliance system that had been established among the great powers. This system contributed to the rapid escalation of the conflict following the assassination.
While knowledge of the Schlieffen Plan and the nationality of the assassin can provide context, the collapse of the alliance system is more directly tied to understanding how the assassination led to a larger conflict like World War I. The role that East African soldiers played is less relevant to this specific investigation.