The correct answer is D. Oracle.
An operating system is a software that manages computer hardware and software resources and provides common services for computer programs. It is a vital part of every computer system, and without it, the computer would not be able to function.
DOS, Linux, and Windows are all operating systems. DOS is a discontinued operating system that was developed by Microsoft in the 1980s. Linux is a free and open-source operating system that was developed in the 1990s. Windows is a proprietary operating system that was developed by Microsoft in the 1980s.
Oracle is a database management system (DBMS) that is used to store and manage data. It is not an operating system.