The correct response is The alliance system.
The alliance system was a significant factor that led to the involvement of France, Germany, and Russia in World War I. When Austria-Hungary declared war on Serbia following the assassination of Archduke Franz Ferdinand, Russia mobilized to support Serbia due to their alliance. This action triggered Germany's alliance with Austria-Hungary against Russia, which led France to mobilize in support of Russia, ultimately resulting in a wider conflict and the outbreak of the war.