The answer is (usted) because you dont have a personal relationship with this person.
ustedes means you guys
vosotros means all of us
tu is used for someone whom you are close with like a family member or someone whom you have know for a while.
Answer
You could just go to translate and type in and it will give you the answer.
Answer:
I have been studying Spanish since I was younger but no lol I just go with it
