The part of OS that remains in memory permanently is called

Concept: An operating system is organized into a core resident part and several loadable or user-level components. The kernel is the portion of the OS that is loaded into main memory at boot and stays resident throughout system operation, because it must continuously manage the CPU, memory, and I/O devices and service interrupts and system calls on demand.

Application: Among Shell, Command Interpreter, API, and Kernel, only the kernel matches the description "the part of the OS that remains in memory permanently" — the other three are either user-level programs that can be started and stopped, or an interface specification rather than a resident program.

Contrast with the other options:

• Shell: a command-line or graphical interface program that a user can launch, replace, or exit; it is not a fixed, always-loaded part of the OS.

• Command Interpreter: the component that reads and executes typed commands, often implemented as the shell itself; it too runs as a user-level process rather than a permanently memory-resident module.

• API: a specification of the calls and protocols by which programs request OS services; it is a programming interface, not a piece of code that must sit in memory at all times.

Cross-check: standard operating-system textbook definitions (e.g., Silberschatz, Operating System Concepts) describe the kernel as the part of the OS that is always running and resident in main memory — this matches only the Kernel option.

Hence, the correct answer is Kernel — the always-resident core of the operating system.