How should I adress (grammatically) a referee whose gender is unknown? I'm responding to an anonymous referee who made comments and suggestions on an article I wrote.

Since I don't know the gender, I thought of two possibilities:


We agree with the referee on its comment...
We agree with the referee on their comment...


Is either of these the proper way to refer to the referee? Or should I just write "he/she" "his/her" on every occasion?
The use of singular they, as in your second option,


We agree with the referee on their comment...


is perfectly appropriate. You can also use some other alternatives, most notably he/she, as in e.g.


We agree with the referee on his/her comment...


I would discourage you from using some of the more niche alternatives, such as Spivak pronouns, in correspondence that's this formal.

On the other hand, your first alternative (We agree with the referee on its comment...) should not be used under any circumstances. It is considered extremely rude by native English speakers, since it reduces the referee to the level of an inanimate thing. If you use it you can hope, at best, to come off as someone with a very poor command of the English language.