Answer:
C. The French attack on Germany was caused by the military alliance is your best answer.
Step-by-step explanation:
Note that it says that France & Russia had a military alliance. A military alliance is an alliance in which the two countries agree to help each other in the event that one or the other gets attacked. To honor their alliance, when Russia was attacked, France had to declare war (which they did). This is shown at the end of the excerpt given to you (both French and Russian troops were battling the German military).
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