Which of the following is NOT a primary function of an operating system?

The correct choice, which identifies the option that is not a primary function of an operating system, is designing software. An operating system primarily serves to manage hardware resources, provide user interfaces, and facilitate the execution of applications.

Managing hardware resources involves overseeing the computer's hardware components, such as the CPU, memory, and storage devices, ensuring that they work together efficiently. This management is fundamental to the operation of the computer, as it allows the operating system to allocate resources where they are needed and control hardware access.

Providing user interfaces is also a critical function of an operating system. It creates an environment where users can interact with the computer system, typically through graphical user interfaces (GUIs) or command-line interfaces (CLIs). This interaction is essential for users to perform tasks effectively.

Running applications is another core responsibility. The operating system provides the necessary environment for applications to execute, including loading programs into memory and managing their operation during execution. It ensures that applications can function properly with the resources they require.

Conversely, designing software is not a function of the operating system. Software design typically falls under the realm of application development and programming, where developers create software to run on the operating system rather than being a direct function of the operating system itself.