You should replace Linux/Unix by Linux and add a fourth option "Other Unix-like OS" or "Other *nix". It would be more clear since both OSX and Linux are Unix-like OSes. http://upload.wikimedia.org/wikipedia/commons/7/77/Unix_hist...
So it would be Unix-like or Windows. Eh, and other.
EDIT: I don't agree with your classification. Sorry, Monday morning before my first coffee.
The classification is arbitrary, definitely. I'm not using "Linux" but Ubuntu, in the same way Chrome OS is a valid answer instead of just "Linux".