(A) ** Belgium
(B) France
(C) Switzerland
(D) Luxembourg
EXPLANATIONS BELOW
Concept note-1: -Of course, Belgians refused to let them through, so the Germans decided to enter by force and invaded Belgium on Aug. 4, 1914. By doing so, they violated the Treaty of London, which is why Great Britain, that was bound to guard the neutrality of Belgium, entered the war.
Concept note-2: -Britain and Poland refused. The situation became deadlocked. The German invasion of Poland began at 4.45am on 1 September.
Concept note-3: -The invasion of Belgium or Belgian campaign (10–28 May 1940), often referred to within Belgium as the 18 Days’ Campaign (French: Campagne des 18 jours, Dutch: Achttiendaagse Veldtocht), formed part of the greater Battle of France, an offensive campaign by Germany during the Second World War.
Concept note-4: -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-5: -German troops overran Belgium, the Netherlands, Luxembourg, and France in six weeks starting in May 1940.