Final answer:
The assassination of Archduke Franz Ferdinand by a Serbian nationalist was the action by a national government that started the hostilities leading up to World War I. Austria-Hungary's subsequent declaration of war on Serbia, underpinned by alliances and national interests, triggered a chain reaction among European powers, escalating into a full-scale war. The first few months of the war saw a rapid spread of conflict due to these alliances. So the correct option is A.
Step-by-step explanation:
The Assassination of Archduke Franz Ferdinand and the Outbreak of World War I
The action taken by a national government that started the hostilities leading up to World War I was a. The assassination of Archduke Franz Ferdinand. This event triggered a chain reaction among European powers, due to existing alliances and rivalries. The archduke's assassination by a Serbian nationalist in Sarajevo on June 28, 1914, led Austria-Hungary to issue an ultimatum to Serbia, which Serbia partially rejected. Supported unconditionally by Germany (often referred to as the "blank check theory"), Austria-Hungary declared war on Serbia, resulting in Russia mobilizing to support Serbia. Germany then declared war on Russia, followed by its declaration of war on France. Great Britain entered the war after Germany's invasion of neutral Belgium, thus escalating the conflict into a full-scale war that involved many world powers.
The first few months of World War I saw a rapid expansion of the war as alliances triggered various nations to declare war on each other. The underlying causes of the war, such as nationalism, opposition to foreign rule, and the complex web of alliances, set the stage for a global conflict that had been simmering for years. The conflict that began in the Balkans soon spiraled into what would become known as the Great War.