Final answer:
Austria declared war on Serbia in 1914 because Serbia rejected one of the terms in Austria's ultimatum, demanding Serbian subservience after the assassination of Archduke Franz Ferdinand. Germany's unconditional support to Austria-Hungary and Russia's pledge to defend Serbia escalated the conflict, leading to a global war. Therefore correct option is A
Step-by-step explanation:
Austria declared war on Serbia in 1914 because Serbia rejected one of the terms in Austria's ultimatum, which demanded Serbian subservience to atone for harboring the terrorist organization that planned and carried out the assassination of Archduke Franz Ferdinand.
Additionally, Austria-Hungary felt honor bound to issue a list of demands to Serbia demanding an investigation and various measures to prevent future attacks on their country by anarchists and Serbian nationalists. Although Serbian officials agreed to nearly every demand, German leaders pushed Austria-Hungary to declare war on Serbia.
Russia pledged its support to Serbia, and Germany promised unconditional military support for Austria-Hungary if Russia or any other nation joined the conflict. The eventual German invasion of Belgium drew Great Britain into the war, followed by the attack of the Ottoman Empire on Russia, plunging Europe into a global war.