(A) imperialism.
(B) propaganda.
(C) ** the alliance system.
(D) Germany.
EXPLANATIONS BELOW
Concept note-1: -The war might have involved only two nations, Austria-Hungary and Serbia, if not for. the American Expeditionary Force.
Concept note-2: -The Triple Alliance was originally composed of Germany, Austria–Hungary, and Italy, but Italy remained neutral in 1914.
Concept note-3: -World War I had two main factions: the Central Powers, which included Germany, Austria-Hungary, Bulgaria and the Ottoman Empire; and the Allies, which included France, Great Britain, Russia, Italy, Japan and later, the United States.
Concept note-4: -Creating a defensive atmosphere and the reassurance that one country would be supported by their alliance if they were to engage in conflict. These reasons encouraged the fighting countries to seek conflict rather than peaceful resolution when tensions were at their highest.
Concept note-5: -There were two major alliances leading up to World War One: the Triple Alliance, which included Germany, Austria-Hungary and Italy; and the Triple Entente, which included England, France, and Russia.