Final answer:
The assassination of Archduke Franz Ferdinand of Austria-Hungary was the immediate cause of World War I. Gavrilo Princip, a Serbian nationalist, shot and killed Ferdinand and his wife, leading to a chain reaction of alliances and declarations of war.
Step-by-step explanation:
The spark that was the immediate cause of World War I was the assassination of Archduke Franz Ferdinand of Austria-Hungary on June 28, 1914. Gavrilo Princip, a Serbian nationalist, shot and killed Ferdinand and his wife during their visit to Sarajevo, Bosnia. This event triggered a chain reaction of alliances and declarations of war, ultimately leading to the outbreak of World War I.