The correct answer is option 'C', System.

Explanation:
An operating system (OS) is a type of system software that manages computer hardware and software resources and provides common services for computer programs. It is the most common type of system software because it is the fundamental software that allows a computer to operate and perform various tasks.

Operating systems are responsible for managing the computer's resources, including the central processing unit (CPU), memory, storage devices, and input/output devices. They provide an interface between the user and the computer, allowing users to interact with the computer and run applications.

Key points:

1. Definition of operating system: An operating system is a type of system software that manages computer hardware and software resources and provides common services for computer programs.

2. Function of operating system: The primary function of an operating system is to manage the computer's resources and provide an interface between the user and the computer.

3. Managing hardware resources: The operating system is responsible for managing the computer's hardware resources, including the CPU, memory, storage devices, and input/output devices. It allocates these resources to different applications and ensures their efficient utilization.

4. Providing common services: Operating systems provide common services that are used by applications, such as file management, process management, memory management, and device management. These services allow applications to run smoothly and interact with the hardware.

5. User interface: The operating system provides a user interface that allows users to interact with the computer. This can be a command-line interface, a graphical user interface, or a combination of both.

6. Running applications: Operating systems are responsible for loading and executing applications. They provide a platform on which applications can run and manage the execution of multiple applications simultaneously.

In conclusion, the operating system is the most common type of system software as it manages computer hardware and software resources, provides common services for applications, and allows users to interact with the computer.