(A) Luxembourg
(B) France
(C) Spain
(D) ** Belgium
EXPLANATIONS BELOW
Concept note-1: -It was this action that technically caused the British to enter the war, as they were still bound by the 1839 agreement to protect Belgium in the event of war.
Concept note-2: -Germany, as part of a massive attack on France, invaded northern France through Belgium early on the morning of 4 August. The Belgians called upon Britain for military assistance under the 1839 treaty, and in response London gave Berlin an ultimatum which expired at 11 pm London time, which was ignored.
Concept note-3: -So what caused Britain to enter the Great War? The British came in partly because of Belgium, a neutral state when Germany invaded it (and Luxembourg) as part of the Schlieffen Plan at the start of World War One.
Concept note-4: -Germany declared war on France. To avoid the French fortifications along the French-German border, the troops had to cross Belgium and attack the French Army by the north. Of course, Belgians refused to let them through, so the Germans decided to enter by force and invaded Belgium on Aug. 4, 1914.
Concept note-5: -The assassination of Austrian Archduke Franz Ferdinand (June 28, 1914) was the main catalyst for the start of the Great War (World War I). After the assassination, the following series of events took place: July 28-Austria declared war on Serbia.