On systems with multiple operating systems, the decision to load a particular…

Concept

During startup, firmware hands control to boot software. A boot loader locates a bootable operating-system kernel, loads it into memory, and transfers execution to it.

On a multiboot system, the same boot stage can present configured operating-system entries so that one kernel is selected before it is loaded.

Application

The question asks which component makes the operating-system choice at boot time. The component with boot entries and responsibility for loading the chosen kernel is the boot loader.

Contrast

• Bootstrap names the startup process or initial routine more generally; it is not the specific multiboot selection component named here.

• A process control block records the execution context and management data of a process after the operating system is managing processes.

• A file control block records file metadata for file-system operations.

• A boot loader provides the startup-stage path from a selected boot entry to the corresponding operating-system kernel.

Cross-check and result

If the user selects an operating system from a GRUB-style startup menu, the boot loader loads or chain-loads that system. Therefore, the answer is boot loader.