The correct answer is D: "the severe Russian winter".
This Napoleonic campaign that attempted to invade Russia in 1812, ended up becoming  a turning point in the Napoleonic Wars. The reputation of the Grande Armée was severely damaged after this episode, and ultimately supposed the beginning of the end of the French hegemony in the European continent and of important changes in politics within this continent.
The French army was retreating to Poland when winter started bringing big troubles for the French army. No food was available for the horses, as all land was covered with snow, and hypothermia problems affected both soldiers and animals. All this caused a loss in the cohesion and discipline within the army.
