To investigate why the assassination of Archduke Franz Ferdinand sparked World War I, the most relevant factor from your list would be the collapse of the alliance system.
The assassination of the Archduke in June 1914 set off a chain reaction among the existing alliances in Europe, leading to a rapid escalation of tensions. Other factors, such as the knowledge of the Schlieffen Plan (which was Germany's military strategy for a two-front war), could also provide context but are not directly related to the immediate implications of the assassination itself.
The nationality of Archduke Franz Ferdinand’s assassin and the role of East African soldiers, while interesting in their own right, are less relevant to understanding the reasons for the broader conflict that ensued. Therefore, focusing on the collapse of the alliance system would be the most effective way to investigate the assassination's impact on the outbreak of World War I.