Which event sparked the onset of World War I?

A.

Germany's invasion of Belgium

B.

Great Britain's declaration of war on Germany

C.

the mobilization of Russia's armies

D.

the assassination of the heir to the Austro-Hungarian throne

E.

Austria's seizure of Bosnia and Herzegovina

Germany's invasion of Belgium sparked the onset of World War I

Explanation:

The reason why world war 1 started was that Germany invaded and entered Belgium. Because it wanted to attack France from the northern side. Also it had to go through and cross the borders of Belgium but this was out rightly rejected by the officials of Belgium.

So as a result of this refusal, Germany decided to invade and enter Belgium by force also on 4th of August in the year 1914 and decided to attack France also there after.