Answer : Kernel
A system call, sometimes referred to as a kernel call, is a request by an active process made via a software interrupt for a service performed by the kernel.
The system call interface is the basic interface between a program and the kernel
A kernel is the central part of an operating system. It manages the tasks of the computer and the hardware - most notably memory and CPU time. There are two types of kernels: A microkernel, which only contains basic functionality; A monolithic kernel, which contains many drivers.