Solution

System calls serve as the primary mechanism by which user applications request services from the operating system's kernel. They provide an essential interface between the application layer and the hardware or system resources, enabling tasks such as file management, process control, and network communication. For example, when an application needs to read a file, it invokes a system call, which then passes the request to the operating system. The operating system processes this request and returns the necessary data or confirmation to the application. This functionality is vital for resource management and system integrity, as system calls ensure that applications do not directly interact with hardware resources, thus maintaining security and stability. System calls bridge the gap between user-mode applications and kernel-mode operations, allowing applications to perform complex tasks without needing to understand the underlying hardware intricacies. Option A (User applications communication) - System calls do not facilitate direct communication between applications; instead, they allow applications to interact with the operating system. Option C (Commands executed by the kernel) - While system calls are invoked by user applications to communicate with the kernel, they are not commands executed by the kernel themselves. Option D (User-defined functions) - System calls are not user-defined; they are predefined functions provided by the operating system for application use. Option E (Inter-process communication) - System calls can facilitate inter-process communication (IPC) but are not themselves a form of IPC.