The correct answer is D) Austria declared war on Serbia.
After the assassination of the Austrian archduke, what happened first was that Austria declared war on Serbia.
Archduke Franz Ferdinand and his wife Sophie were assassinated on June 28, 1914, when they were about to visit the Town Hall in Sarajevo. The assassin was Garilov Princip. The agression represented a direct attack to the Austria-Hungary empire and this incident was one of the main reasons of World War 1. So it is correct to say that after the assassination of the Austrian archduke, what happened first was that Austria declared war on Serbia.
