"A day after Austria-Hungary declared war on Serbia, Russia ordered mobilization against Austria-Hungary." Therefore, option (b) is the correct answer.
The true sequence of events is that once Austria-Hungary declared war on Serbia, Russia mobilized its forces, which led Germany to declare a two-front war against Russia and France.
When Austria-Hungary declared war on Serbia, it also signaled the start of World War I (WWI), which was eventually recognized as one of the world's worst and most destructive incidents.
The ordering of the mobilization of its forces by communist Russia was the cause of this declaration of war, which accelerated the start of the war between Germany and nations like Russia and France.
Therefore, out of the other choice, option (b): "Russia," is the correct answer.
Check out the link below to learn about Austria-Hungary war;
https://brainly.com/question/17120913
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