Which of the following is NOT a function of an Operating System?

The correct answer highlights that application programming is not a core function of an operating system. Instead, application programming generally refers to the process of creating software applications that run on top of the operating system. An operating system's primary role revolves around managing hardware and software resources on a computer.

File management involves overseeing how data is stored and accessed on disk drives, ensuring that users can create, read, write, and delete files efficiently and securely. Process scheduling is a critical function that refers to how the operating system allocates CPU time to various running applications and processes, ensuring optimal performance and responsiveness. User authentication is vital for security, verifying the identity of users to protect system resources and data.

By focusing on resource management and security, the operating system creates a stable environment for applications, which are built and run separately from the OS itself. Therefore, application programming does not fall under the direct purview of what an operating system does.