The correct answer is D. Which
Explanation:
Which is a pronoun and determiner that it is used in sentences to refer to something that was previously mentioned, in this case, it is "the winter". Additionally, according to grammar "which" is used for connecting clauses to the main sentence, specially clauses that are not necessary for the main sentence (dependent clause), in this case the main sentence is "the winter seemed to last forever" and the depended clause is "which was long and brutal", this can be considered as a depended clause as it can be omitted and the sentence will still have a complete meaning, thus, as "was long and brutal" refers to the winter the only options would be that or which, but considering this is a dependent clause the correct option is which.
