Brit Linux is indeed Unix, thus you are correct. But Unix is hardly ever heard of these days, and so I would have excepted Linux in place of Unix. MS-DOS is indeed the Grand Daddy of Windows 7 and I accept that MS-DOS still lives with Windows 7. And of course Apple OS is..... well I have no idea what the latest flavour is, because I don't have an apple.
Yes I know there are other specialist Operating Systems out there, but these are the three core domestic OS's.