When Austria-Hungary declared war on the Slavic people of Serbia, Russia began to prepare to send troops to support Serbia.
Russia:
As a result of Russia preparing to send soldiers, Germany got involved and declared war on Russia which led to France declaring war on Germany and Great Britain being dragged into what became the First World War.
In conclusion, option C is correct.
Find out more about Russia's actions at https://brainly.com/question/2628215.