Final answer:
In French, the subject pronoun to address a classmate is 'tu'.
Step-by-step explanation:
The correct subject pronoun to address a classmate in French is tu. This pronoun is used informally when speaking to a single classmate or a friend. The pronoun vous is used to address classmates or multiple people formally. For example, if you were addressing your classmate Jean, you would say "Salut Jean, comment vas-tu?" meaning "Hi Jean, how are you?"
Learn more about Subject pronouns in French