Work at/in

If you'll tell us which one you feel is right, we'll tell you if you are correct.

I think all of them are correct and available, but I guess there might be some difference in terms of meaning between each of them. That's why I posted this question. For example, I know you would say, "I have to arrive at the airport before departure" when you need to go abroad by plane, but I am not sure whether you could say, "As a maintenance technician, he works [ (A) at airport / (B) a

In my opinion, only B is correct but I can't say why. As far as I know it's a fixed expression at + the + airport. It's always been a mystery to me why we have to use the definite aritcle there though.

Thank you. I agree that B is the correct one.
Airports are a combination of external and internal facilities, so they have generally been referred to as “at” rather than “in”. We also seem to prefer to exclude the likelihood that the particular airport needs to be named—possibly because there is usually only one nearby—so “the airport” is used instead of “an airport”. I hope that helps.