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The appearance of the progress bar signifies the transition to the Kernel initialization step during the boot process of a Mac. This visual element indicates that the system has successfully completed the preceding steps, such as power-on self-tests and firmware checks, and is now transitioning to load the operating system's kernel. The progress bar represents the active loading phase where the system is engaging with the kernel and initializing system processes and services necessary for the operating system to function.

The other options relate to different stages of the boot process. The appearance of the apple logo indicates that the system has moved beyond the initial power-on tests and firmware loading, but it does not mark the specific transition to Kernel initialization. Completion of firmware tests occurs before any noticeable user interface elements display, and though loading of kernel extensions is a crucial part of the kernel operation, it happens after the Kernel itself has been initialized. The progress bar serves as a clear visual marker that the system is now preparing to launch the operating system and is actively engaging the kernel.