An operating system (OS) is a crucial software component that acts as an interface between the hardware and application programs on a computer system. It is responsible for managing and controlling various hardware resources, providing system services to application programs, and ensuring the overall functionality and performance of the system.

The correct answer for this question is option 'D' - "All of the mentioned", as an operating system performs all the mentioned tasks.

Interface between the hardware and application programs
- One of the primary functions of an operating system is to provide a user-friendly interface that allows users to interact with the computer system. It provides a platform for running application programs, manages input and output devices, and handles user requests.

Collection of programs that manages hardware resources
- An operating system manages various hardware resources such as the CPU, memory, disk drives, and peripherals. It allocates these resources to different processes and ensures their efficient utilization. The OS also handles tasks like process scheduling, memory management, file system management, and device drivers.

System service provider to the application programs
- An operating system provides a range of system services to application programs. These services include handling input and output operations, managing files and directories, providing network communication capabilities, and ensuring security and protection of the system. The OS acts as an intermediary between the applications and the hardware, abstracting the complexities of the underlying hardware and providing a uniform interface for application development.

In summary, an operating system serves as the crucial link between the hardware and application programs on a computer system. It provides a user-friendly interface, manages hardware resources, and offers system services to application programs. Hence, the correct answer for this question is option 'D' - "All of the mentioned".