For me, to find if something is an adverb, I just replace the word in question with the word "quickly" to see if it makes any sense. If it still makes sense, then the word in question is an adverb.
This is because due to the verb in accordance to 'down', removing the adverb would not make a complete sentence. Since a verb is used to describe an action, the adverb would only come after a verb in order to describe it where necessary. Therefore, D. is the answer.
