Question

Which geographic factor was most significant in

Answer 1

There were of course several geographic factors that were significant in helping the Soviet Union withstand German attacks in World War II, but the best option is "(2) Distance and harsh winters disrupted German supply lines."

Answer 2

I would say (2)
Long distances through the Russian steppes elongated the communication and supply lines of the German Army requiring almost the same numbers of men to man them as the ones to actually fight the war (most of them used to fight partisans attacking supply lines). The harsh winters deteriorated even more this situation disrupting mechanized and horse transportation and producing the need of even more specialized goods (anti-freeze, furs, special clothes, etc.).