The correct answer to this question is D, Adverb clause. Adverb clauses are also known as adverbial clauses. It is a clause that has the characteristic of being dependent, but it can also function as an adverb. A way to identify an adverb clause is to identify subordinate conjunction at the beginning of the sentence.
The subordinate conjunction is also known as a trigger word. If it is at the beginning of the sentence, it is more than likely an adverb clause, because most of them begin with it. There are different types of adverb clauses, including time, which answers the when and such as, and condition, which answers the if and lest.