Sounds deliberately tricky to me. For me, the passage is juxtaposing the cold Sapporo winter with the fun winter matsuri. Therefore, the answer could potentially be
a. The matsuri is so fun, we can forget how cold it is OR
b. The winter is so cold, we tend to forget that there are fun things to do even in winter.
But I think the answer implies volition to forget, so A would be the most appropriate. It is possible for us to (willfully) forget how cold it is.
So それ refers to 札幌は寒くて大変
That's my interpretation anyway. The period (。) after ですから seems to make the question deliberate misleading.