Kernel.
Explanation:
An operating system platform is a system software pre-installed on a computing device to manage or control software application, computer hardware and user processes.
This ultimately implies that, an operating system (OS) is essentially an interface between the computer’s hardware and all the software applications (programs) running on it.
Some examples of an operating system are QNX, Linux, OpenVMS, MacOS, Microsoft windows, IBM, Solaris, VM, etc.
Kernel is part of the operating system core. It is responsible for controlling security, managing the file system, and providing a platform for applications to run on.
Basically, the kernel is the foundational or fundamental layer of the operating system (OS) and as such it is loaded into memory whenever the computer system starts to boot i.e the kernel must always reside in the memory of a computer because it is saddled with the responsibility of managing computer resources such as the central processing unit (CPU), peripheral devices, random access memory (RAM) and input/output instructions from software programs.