1.  What is kernel?

The kernel is the essential foundation of a computer's operating system (OS). It is the core that provides basic services for all other parts of the OS. It is the main layer between the OS and underlying computer hardware, and it helps with tasks such as process and memory management, file systems, device control and networking.

2. Is Linux an operating system or a kernel?

Linux is just a kernel, not an operating system. The kernel is at the center of every operating system. It's not just a Linux thing. Windows and macOS have kernels too.

3.   Is Linux Operating System Immune to Malware?
There is no operating system which is not attacked by immune virus. Here Linux also have possibility of malware but it is secure than Windows.