The best answer is A
The Soviet army won that battle due to a number of factors that worked in their favor. They won it without any help from the allied forces, who were engaged in other battle fronts.
The Germans were not prepared for the bitter Russian winter. They still operated in fatigues and boots meant for spring and summer fighting and suffered serious frost bite, killing their morale, while the Russians were warmly dressed in fur skinned gear. German vehicles often got bogged down in mud, and sometimes engines froze so mules and horses had to be used for transport.
Hitler's refusal to heed his commanders' advice resulted in the near annihilation of the 6th Army that had spear headed the attack on Stalingrad.
The Germans made the mistake of engaging in Urban warfare, street by street, building by building, in Stalingrad in the bitter freezing temperatures where the soviets had the advantage.
Russian ability to replace their losses man for man resulted in them mastering a massive army that caught the German divisions in a pincer maneuver finally surrounding them forcing the Germans to surrender.
