Which one of the following is not a layer of operating system?

The correct answer is D. Critical selection.

An operating system (OS) is a system software that manages computer hardware and software resources and provides common services for computer programs. The OS acts as an interface between a user and the computer hardware, and it also manages the execution of all other programs on the computer.

The OS is divided into four layers:

• The kernel is the lowest layer of the OS. It is responsible for managing the computer's hardware resources, such as the CPU, memory, and storage.
• The shell is the next layer up. It provides a user interface for interacting with the OS.
• The application program layer is where user programs run.
• The system service layer provides services to both the application programs and the shell.

Critical selection is not a layer of the OS. It is a process that is used to select the best candidate for a job or position.