Final answer:
The word 'before' in the sentence functions as an adverb, modifying the verb 'arrived'.
Step-by-step explanation:
In the sentence 'Melissa's cousin arrived at the restaurant before Melissa,' the word 'before' functions as an adverb. It modifies the verb 'arrived' by specifying the time relationship between Melissa's cousin's arrival and Melissa's arrival. In English, a word can serve different parts of speech depending on its use within a sentence. However, in this case, 'before' is used to compare two events in time, indicating that it is serving as an adverb, not a noun, verb, or adjective.