Question

Arctic winters, which are long and cold, are severe

Answer 1

The correct answer is C. it is a nonessential adjective clause.

The adjective clause which are long and cold is separated from the rest of the sentence using commas, which means that it is nonessential - if it were omitted from the sentence, it would still make complete sense.

This means that D is incorrect - if there were no commas, then it would be essential. A and B are also incorrect - the clause is actually connected by a relative pronoun which, rather than an adverb or a conjunction.

Answer 2

The correct answer is (it is a nonessential adjective clause)