Which of the following was an immediate cause of World War I?

The assassination of Archduke Franz Ferdinand is recognized as the immediate cause of World War I because it directly triggered a series of events that led to the outbreak of the war. This event, which occurred on June 28, 1914, set off a chain reaction involving alliances and diplomatic tensions among European powers. After the assassination, Austria-Hungary issued an ultimatum to Serbia, which was only partially accepted. This led Austria-Hungary to declare war on Serbia, and due to the complex web of alliances, other nations soon became involved, leading to a full-scale conflict.

While militarism, imperialism, and nationalism were significant long-term causes contributing to the tensions leading up to the war, they were not the immediate events that initiated the military conflict. The assassination act provided the spark that ignited these underlying tensions into warfare, making it the crucial catalyst for World War I.