The Answer:

The correct answer is option 'B' - System call.

Explanation:

To understand why the system call provides the interface to access the services of the operating system, let's break down the various options:

1. API (Application Programming Interface):
- An API is a set of rules and protocols that allow different software applications to communicate with each other.
- While APIs are commonly used to access various services and functionalities, they are not specific to the operating system.
- APIs can be provided by libraries, frameworks, or other software components.

2. Library:
- A library is a collection of precompiled functions and routines that can be used by software applications.
- Libraries can provide access to certain functionalities or services, but they do not necessarily provide the interface to the operating system itself.
- Libraries can be utilized by developers to simplify their coding tasks or access specific functionalities.

3. Assembly instruction:
- Assembly instructions are low-level instructions that directly manipulate the hardware components of a computer.
- Assembly language is specific to the hardware architecture and is not a direct interface to the operating system services.

Now, let's focus on the correct option:

4. System call:
- A system call is a mechanism provided by the operating system that allows user programs or processes to request services from the kernel.
- It acts as an interface between the user-level programs and the operating system kernel.
- When a program requires access to specific operating system services (such as file operations, network communication, or process management), it makes a system call to request the required service.
- The system call transfers control from the user-level program to the operating system, which performs the requested operation and returns the result to the program.

Therefore, system calls provide the necessary interface for user programs to access the services and functionalities provided by the operating system.