Final answer:
Russia came to the aid of Serbia following the assassination of Archduke Franz Ferdinand, leading to the outbreak of World War I.
Step-by-step explanation:
Following the assassination of Archduke Franz Ferdinand, the nation that came to the aid of Serbia was Russia. This is because of a treaty in which Russia had agreed to defend Serbia, as well as historical connections and Slavic nationalism. Serbia's support from Russia encouraged them to resist Austro-Hungarian demands after the assassination, which in turn led Austria-Hungary, backed by Germany's "blank check," to declare war on Serbia on July 28, 1914, triggering World War I. This conflict of alliances quickly spiraled into a global conflict, drawing nations into war according to their treaty obligations.