(A) signing of the Treaty of Versailles
(B) invasion of Poland by Germany
(C) ** assassination of Archduke Francis Ferdinand
(D) use of unrestricted submarine warfare by Germany
EXPLANATIONS BELOW
Concept note-1: -The assassination of Archduke Franz Ferdinand was the immediate cause of a war between two great powers-Russia and the Habsburg Austro-Hungarian Empire.
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: -Just hours after narrowly escaping an assassin’s bomb, Archduke Franz Ferdinand, the heir to Austro-Hungarian throne and his wife, the Duchess of Hohenberg, are killed by Gavrilo Princip. A month later, Austria-Hungary declares war on Serbia and Europe rapidly descends into chaos.