**Answer:**

The correct answer is option B: **System call**.

**Explanation:**

**Interface to Access Services of the Operating System:**

The interface that provides access to the services of an operating system is a system call. A system call is a mechanism provided by the operating system that allows user programs to interact with the operating system kernel. It provides a way for user programs to request services from the operating system, such as accessing files, allocating memory, creating new processes, etc.

**Other Options:**

Let's briefly discuss the other options mentioned:

a) **API (Application Programming Interface):**
An API is a set of rules and protocols that allows different software applications to communicate with each other. It provides a set of functions, procedures, and protocols for building software applications. While an API may be used to access the services of an operating system, it is not the direct interface provided by the operating system itself.

c) **Library:**
A library is a collection of precompiled code and resources that can be used by software programs. Libraries often contain APIs that provide access to various functions and services. While libraries may provide access to the services of an operating system, they are not the direct interface provided by the operating system itself.

d) **Assembly Instruction:**
Assembly instructions are low-level instructions written in assembly language that are directly executed by the processor. They are used to write programs at a very low level, closer to the hardware. While assembly instructions may be used to interact with the operating system, they are not the primary interface provided by the operating system itself.

**Conclusion:**

In conclusion, the interface provided by the operating system to access its services is a system call. System calls allow user programs to interact with the kernel of the operating system and request various services.

**Explanation:**

The interface that provides access to the services of the operating system is known as a system call. A system call is a mechanism that allows user-level programs to interact with the operating system kernel.

**System Call:**

A system call is a request made by a program to the operating system for performing a specific task or accessing a specific service. It acts as a bridge between user-level programs and the kernel of the operating system.

**Interface to Operating System:**

The operating system provides various services and functionalities to user-level programs, such as file operations, process management, memory management, device management, and more. To access these services, user-level programs need to use system calls.

**API (Application Programming Interface):**

While an API (Application Programming Interface) also provides an interface to access the services of the operating system, it is a higher-level interface provided by libraries or frameworks. APIs are typically designed for specific purposes, such as networking, graphics, or database access. They may internally use system calls to interact with the operating system.

**Library:**

A library is a collection of precompiled functions or routines that can be used by programs to perform specific tasks. Libraries can provide a higher-level interface to access the operating system services. They may internally use system calls or APIs.

**Assembly Instruction:**

Assembly instructions are low-level instructions that are directly executed by the processor. They do not provide a direct interface to access the services of the operating system. However, system calls can be invoked using specific assembly instructions, such as INT (interrupt) or SYSCALL.

**Conclusion:**

Among the given options, the system call is the correct answer as it provides the interface to access the services of the operating system. APIs, libraries, and assembly instructions can also be used to interact with the operating system but they are not the primary interface.