The correct answer is "vous." This is because "vous" is the formal and polite form of addressing someone in French. It is used when speaking to someone who is older, in a position of authority, or someone you do not know well. "Tu," on the other hand, is the informal form used when speaking to friends, family, or people of the same age or younger. Therefore, when addressing a teacher, it is more appropriate to use "vous" to show respect and politeness.