Question

which event had the immediate effect of causing germany to declare war on the allies?
the berlin conference of 1885 sets off the \scramble for africa.\
russia mobilizes its forces on germanys eastern borders.
great britain declares war on germany.
the austrian archduke is assassinated.

Explanation:

The mobilization of Russian forces on Germany's eastern borders was a direct threat that led Germany to declare war on the Allies. The Berlin Conference was about Africa - not related to Germany's declaration of war on Allies. Great Britain declaring war on Germany was a result of Germany's actions, not the cause of Germany declaring war on Allies. The assassination of the Austrian archduke was the spark that led to the start of World War I but not the immediate cause of Germany declaring war on the Allies.

Answer:

Russia mobilizes its forces on Germany’s eastern borders.