C. is the correct use of "down" as an adverb. "Ricky rolled 'down' the car window to place his order at the coffee shop." Because "down" is used to signify the lowering of an object from an higher position, it is an adverb (adverbs describe verbs). The other options to not require "down" in the sentence in order to make sense. For them, it is a general direction and not part of the verb.