computer question...?

At 99% its the primary Hard Disk with drive letter C:\

If you are talking about personal computers, the most accurate answer would be the hard disk drive.

Now for a history lesson...

A few decades before you were born (likely, ha ha) the operating system was known as the disk operating system aka DOS or MS-DOS (Microsoft Disk Operating System). In those days, you couldn't do much more with the operating system than ummm, operate the various disks (floppy disk, hard drive, later CD-Rom drive). The operating system came on floppy disk or floppy disks, and you would copy it to the hard drive.

Now flash forward to Windows 7 or a later version of linux like ubuntu 9.1. What we call an operating system today does virtually everything that you want to do on the computer. Operating the disks is a minor utility of the operating system now.

So it's not surprising that someone would be asking what storage device holds the operating system. The hard drive and the operating system used to be pretty much totally dependent on each other for their functionality. Now it's getting to the point where you have to wade through several OS menus just to access a hard drive...which will be obsolete within a couple of decades.

99% of the time hard drive.

But you can also store "live" operating systems on any media.. optical discs, flash drives, etc..

Hard drive

Read your text book, that's why you have it.

Hard Drive. BIOS is the name of where it is stored.