The core concept here is to understand the function of 'as' in a sentence. The correct answer is C. Subordinate conjunction because 'as' is used to introduce a subordinate clause that explains the main clause. Option A is incorrect because 'as' is not simply a conjunction; it has a specific role in connecting the clauses. Option B is wrong because 'as' is not a preposition in this context. Option D is incorrect because 'as' is not functioning as an adverb in this sentence.