I'm a native speaker. I would say "Kom deg ut av kontoret mitt!"

As a native i would point to the door and say "Ut!". No point with with being polite if you really want the person to leave ;)

I'm also a learner, but (a) seems the better choice.

The sentence requires use of the imperative form of the verb, which you may not have learned yet, but gå is already in its imperative form, where as komme should be replaced by kom. Also using kom is like telling someone to come out of your office rather than go out, it suggests you yourself are outside the office.