Which version of Windows 7 to buy to legally use in Ubuntu VirtualBox? [closed]

I've read that Windows 7 cannot be virtualized on non-licensed PCs. Ubuntu isn't licensed with Microsoft's license, of course, which means that I cannot use it to virtualize Windows 7. I've read that there are other versions that are "OEM" or "non-OEM" or something like that and which have different terms and conditions.

What is the exact name of the version of Windows 7 that I should buy in order to be legally able to use it in VirtualBox on Ubuntu?

For personal use, the regular "Retail" license is the simplest option.

As far as I know, "OEM" versions are only sold to computer manufacturers (i.e. OEMs) – they're meant to be sold preinstalled in computers, not separately. (Also even if you have an OEM license you're not allowed to move it to another computer, so I'm not sure if that would allow you to run Windows in a VM anyway.)