Which of the following events was the immediate cause of Great Britain's entry into the First World War?

A) The Austro-Hungarian invasion of France
B) The mobilization of the Russian army
C) The German invasion of Belgium
D) The declaration of war against Serbia by Austria-Hungary

Russian Army

is the land-based military force of the Russian Federation, inheriting the traditions and structure from the Soviet Union.

German Invasion

Refers to military incursions or attacks by Germany, especially notable in World War I and World War II contexts.

Austro-Hungarian Invasion

Military operations conducted by the Austro-Hungarian Empire, notably its attacks and occupations during World War I.