Explanation:

The correct option is (c) Operating System. The 'boot' process is the sequence of operations that a computer performs when it is powered on, culminating in the loading of the operating system (OS) into the computer's memory. The Operating System is the core software that manages all hardware and software resources, providing essential services for computer programs. Without an OS, the computer cannot start up, manage its components, or run any applications. Option (a) Compiler and (d) Assembler are language processors that translate code, not essential for the boot process itself. Option (b) Loader is a part of the operating system responsible for loading programs into memory *after* the OS has been initialized. The OS itself must be loaded first for the computer to become functional.