Final answer:
The word "before" in the sentence is a preposition.
Step-by-step explanation:
In the sentence "Melissa's cousin arrived at the restaurant before Melissa," the word "before" is a preposition. Prepositions show the relationship between a noun or pronoun and other words in a sentence. In this case, "before" indicates the relationship of time between the arrival of Melissa's cousin and Melissa.
Learn more about parts of speech