The correct answer is - D) the severe Russian winter.
Napoleon was not the first, nor the last, that made a big mistake in underestimating the weather conditions of Russia, and it was fatal for his army. Russia is well known for its severe weather conditions in winter, but Napoleon thought that he can win the war very quickly and also that the weather will be only a minor problem, and he made a huge mistake. The Russians played very well tactically by making a false retreat further north and deeper into the country, and this lured the French troops easily. The French were caught unprepared by the extreme weather conditions, and they started to get ill and die, lost big portions of their food, the horses started to die as well, and the army was in a situation were it struggled to survive, yet alone to go fight and win a war. The Russian waited wisely, and once the French were weakened enough they attacked and won with relative ease.
A) an advanced Russian military
Napoleon's Grande Armée of 610,000 men invaded Russia, heading towards Moscow, in the beginning of summer on 24 June 1812. The french troops were at the rear and the Russian at the front. Of course there would be little availability of everything. The Russians made the problem worse by adopting a scorched earth policy of destroying everything possible as they retreated before the French. Therefore French troops got weakened and and Napoleon was defeated.
