Final answer:
In the sentence, 'Melissa's cousin arrived at the restaurant before Melissa,' the word 'before' is a preposition indicating the relationship of time.
Step-by-step explanation:
In the sentence, 'Melissa's cousin arrived at the restaurant before Melissa,' the word 'before' is functioning as a preposition. Prepositions typically show the relationship between nouns or pronouns and other words in the sentence. In this case, 'before' is showing the relationship of time between Melissa's cousin arriving and Melissa's arrival.
Learn more about Part of speech in a sentence