Lol. You have the choice of the non-native speakers or a native speaker :)

Hello there! I am not a native but maybe I can be of help. Since what the person is questioning is not if he thinks or not, I would exclude number 1 and also number 4 since "is" is nowhere to be found in the sentence. We are then left with 2 and 3. With the "I don't think" the interlocutor is saying "He hasn't met our teacher" so the tag should be affirmative, so we are felt with number 3)has he. He seems to be looking for reassurance for something he thinks he already knows. Does it make sense?

#2 is the correct answer. The question posed as a negative requires a negative answer of some form.

Hey, You are using "... he has met ..." and also you are using "... do not ...", your first sentence is negative, So ... the answer should have "has" and should be positive, With this information number 3 is right :)

Although I'm not a native speaker, I'll still give it a try :) I'd pick 3, since the sentence is negated.