The correct answer is:
A second assassination attempt succeeded, and Austria-Hungary declared war on Serbia.
In reply to the assassination of Archduke Franz Ferdinand by Garvrilo Princip in Sarajevo on 28 June 1914, Austria-Hungary declared an ultimatum to Serbia, which was partly denied. Austria-Hungary then suspended diplomatic relations and declared war.
In response, and following the Secret Treaty of 1892, Russia and France mobilized their armies. Russia's mobilization set off full Austro-Hungarian and German mobilization.
Soon all the Great Powers, besides Italy, had taken sides on WW1.