(A) Russia wanted to trade with Serbia, so they declared war on Austria-Hungary.
(B) Germany asked Russia to stay out of the war, but Russia refused.
(C) ** The assassination of Archduke Franz Ferdinand by a Serbia nationalist.
(D) the assassination of Woodrow Wilson, America’s president.
EXPLANATIONS BELOW
Concept note-1: -The political objective of the assassination was to free Bosnia and Herzegovina of Austria-Hungarian rule and establish a common South Slav ("Yugoslav") state. The assassination precipitated the July Crisis which led to Austria-Hungary declaring war on Serbia and the start of World War I.
Concept note-2: -The assassination set off a rapid chain of events, as Austria-Hungary immediately blamed the Serbian government for the attack. As large and powerful Russia supported Serbia, Austria asked for assurances that Germany would step in on its side against Russia and its allies, including France and possibly Great Britain.
Concept note-3: -Leading up to the Assassination Serbia was an ally with the powerful Russian Empire and Austria-Hungary was allied with Germany. When Archduke Ferdinand announced his trip to Sarajevo, Bosnian nationalists (people who wanted Bosnia to be its own country) saw their opportunity to strike at the Austrian Empire.