Answer:
Franz Ferdinand assassination
Step-by-step explanation:
The assassination of Franz Ferdinand on June 28, 1914, in Sarajevo, Bosnia, was the immediate trigger for the outbreak of World War I. The assassination was carried out by a Serbian nationalist, Gavrilo Princip, who was part of a group seeking to create a Greater Serbia. The assassination led Austria-Hungary to declare war on Serbia, which then triggered a chain reaction of alliances and declarations of war among the major European powers.
On the other hand, the Australian ultimatum was a diplomatic event that occurred several weeks after the assassination. In early August 1914, the Australian government issued an ultimatum to Germany, demanding that German troops leave New Guinea and surrounding islands. While this ultimatum was significant in terms of Australia's involvement in the war, it did not directly lead to the outbreak of war in Europe.
Therefore, while both events played a role in the causes of World War I, the assassination of Franz Ferdinand is generally seen as the more significant trigger for the outbreak of war.