Final answer:
The correct word to complete the sentence about cousins living in Miami is 'who,' as it is the appropriate subjective case pronoun used for people.
Step-by-step explanation:
The correct option to complete the sentence "Gustavo, Carmen, and Alejandro are my cousins __________ live in Miami." is A. who. The word 'who' is used as a subjective case pronoun that refers to people and acts as the subject of the relative clause. Since Gustavo, Carmen, and Alejandro are the subjects who are performing the action of living (in Miami), we use 'who' instead of 'which' or 'that'. 'Which' is usually used for objects, and 'that' could be used, but 'who' is the most appropriate when specifically referring to people in a nonrestrictive clause.