Final answer:
The assassination of Archduke Franz Ferdinand and his wife in Sarajevo by Gavrilo Princip triggered the complex alliance system of Europe, which led to the outbreak of World War I.
Step-by-step explanation:
The assassination that sparked World War I was that of Archduke Franz Ferdinand of Austria-Hungary, along with his wife Sophie. This event took place in Sarajevo, the capital of Bosnia, on June 28, 1914. The perpetrator was Gavrilo Princip, a member of a secretive Serbian nationalist group known as The Black Hand. Following the assassination, Austria-Hungary issued a series of demands to Serbia, which, when not fully met, led to the Austro-Hungarian declaration of war on Serbia. This set off a chain reaction, activating various European alliances and drawing several nations into a conflict that escalated into the first global war, known as World War I.