D. Preposition > The eight parts of speech (traditional grammar classifies words): the verb, the noun, the pronoun, the adjective, the adverb, the preposition, the conjunction, and the interjection. Each part of speech explains how the word is used (not what the word is). The same word can be a noun in one sentence and a verb/an adjective in the next.
A preposition links pronouns, nouns, and phrases to other words in a sentence. It usually indicates the temporal, spatial or logical relationship of its object to the rest of the sentence. The word/phrase that the preposition introduces is called the object of the preposition.