Final answer:
The scenario where France declared war on Germany immediately after Germany declared war on Russia would suggest that the military alliance between France and Russia was the direct cause of France's entry into World War One.
Step-by-step explanation:
The most likely scenario to suggest that the military alliance between France and Russia was the cause of France entering the war with Germany is A. France declared war on Germany immediately after Germany declared war on Russia. This action would directly reflect the operative terms of the alliance, whereby each country assured mutual support in case one of them was attacked. The alliance effectively bound France to contain the ambitions of Germany.
If France declared war on Germany immediately after Germany declared war on Russia, it would demonstrate that the existing alliance prompted the French to fulfill their treaty obligations by engaging in the conflict. This would be particularly the case given the complex web of alliances among European powers and the German anxiety about a two-front war, which ultimately factored into their decision to launch a preemptive strike.