Unix.
The Unix operating system. The first versions of the Unix operating system were written in the "B" language, and later written in "C", which was invented in order to develop Unix on the PDP-11 machine.
Some parts of Windows may be written in C++, I'm not sure.
An operating system is a program that can be written in C
No. The bulk of the Windows operating system is written in C++.
I guess you mean operating system written in C language. Two examples: Unix (and derivatives), MS Windows.
C/C++ is used to write system softwares. The whole of UNIX operating system is written in C language.
C and C++ are not directly supported by operating systems. You must use a compiler to convert a program written in C or C++ to a native executable that runs under a particular operating system. In other words, you can use C or C++ under any operating system for which a compiler exists for the desired language.
An operating system is mostly developed in C. C++ is less common, but used by several OS developers. There also is some operatingsystems developed in Pascal and Ruby. The last (ruby) used an interpreter in C. To load the operating system, you need a bootloader. Which is at least partially written in assembler.
Because Linux is "sort-of-a-copy-of-Unix" (which is not), and Unix was written in C. C was created to write Unix. Most portable operating systems or serious embedded devices are written in some incarnation of C and C++. Including MS-DOS and earlier copies of Window$ (which I am sure still uses it)
The operating system on a computer is installed on your hard drive, usually under c:
Maybe C. or Assembler. Linux has some RT capability, and it's written in C. I think VxWorks (a commercial RTOS) is written in C also.
The operating system on a computer is installed on your hard drive, usually under c: