(A) Germany
(B) Great Britain
(C) ** Austria Hungary
(D) ** Serbia
EXPLANATIONS BELOW
Concept note-1: -This list is an overview of the most popular reasons that are cited as the root causes of World War 1. Austria-Hungary declared war on Serbia, Russia got involved to defend Serbia. Germany seeing Russia mobilizing, declared war on Russia.
Concept note-2: -On July 28, 1914, one month to the day after Archduke Franz Ferdinand of Austria and his wife were killed by a Serbian nationalist in Sarajevo, Austria-Hungary declares war on Serbia, effectively beginning the First World War.
Concept note-3: -The Serbian campaign was a series of military expeditions launched in 1914 and 1915 by the Central Powers against the Kingdom of Serbia during the First World War.
Concept note-4: -With the decline of the Ottoman Empire, Serbian nationalism continued to rise, culminating in the assassination of the Archduke of Austria in 1914 by a Bosnian Serb and officially triggering the start of the Great War.