The part of speech for the italicized word in the sentence below is a preposition. Option D is correct.
Prepositions are words which start prepositional phrases. A prepositional phrase constists of a group of words which contains a preposition, a noun or pronoun object of the preposition, and any modifiers of the object.
Down is often used with verbs of movement and also in phrasal verbs which usually refers to go towards the ground or to a lower level.
