The correct answer is 1.
The vast Russian territories, together with the very hard weather conditions during the winter, to which foreign armies were not used at all, defeated two armies that until the moment had obtained mostly sucesseses in their invasion attempts: Napoleon's army and the German Nazi army during WWII (Hitler's army came from invading France and most territories in Western Europe and aimed to expand their territories to the East afterwards).
Foreign forces that tried to invade Russia had to stand the winter, together with the very complicated conditions that are present in any case in the camp of an army during a war.