The correct answer is option 'D', operating system software.

Operating system software is a type of software that controls the internal operations of a computer and manages how the computer works with all its parts. It acts as an intermediary between the hardware and the user, providing a user-friendly interface and managing various tasks and resources of the computer system.

Operating system software performs several important functions, including:

1. Hardware management: It controls and manages the hardware components of the computer system such as the central processing unit (CPU), memory, storage devices, input/output devices, and other peripherals. It allocates system resources to different processes and ensures efficient utilization of hardware resources.

2. Process management: It manages and controls the execution of multiple processes or programs running on the computer system. It schedules the processes, allocates system resources, and provides mechanisms for inter-process communication and synchronization.

3. Memory management: It manages the computer's memory by allocating and deallocating memory space for different processes. It keeps track of the available memory and ensures that each process has sufficient memory to execute.

4. File system management: It provides a hierarchical structure for organizing and storing files and directories on the computer's storage devices. It manages file access, permissions, and storage allocation.

5. User interface: It provides a user-friendly interface for interacting with the computer system. It can be in the form of a command-line interface (CLI) or a graphical user interface (GUI) that allows users to interact with the system using visual elements such as icons, menus, and windows.

Some popular examples of operating system software include Microsoft Windows, macOS, Linux, and Unix. These operating systems are designed to work with a wide range of hardware configurations and provide a platform for running various application software.

In conclusion, operating system software is responsible for controlling the internal operations of a computer and managing how it works with its hardware components. It provides essential functions such as hardware management, process management, memory management, file system management, and user interface.

An operating system (OS) is a program or software that manages and coordinates the functions of all the components of a computer. It also acts as an interface between the user and the hardware of a computer.