Which startup key combination is used to start a Mac from a bootable CD?

The key combination used to start a Mac from a bootable CD is the letter "C." When the user holds down the "C" key during the startup process, it directs the Mac to boot from the optical drive if a bootable CD is present. This function is particularly useful for installing macOS or utilizing a recovery disk, allowing the Mac to bypass the internal hard drive and initiate the operating system from the external media.

This option activates the specific firmware setting required to recognize and prioritize the boot from the CD, making it essential in scenarios where the internal drive is not functioning properly, or if a fresh installation is required. Other key combinations have different functionalities that do not support booting from a bootable CD. For example, holding "Option" allows the user to select from all available bootable devices rather than directly booting from a CD. Command-S is used for single-user mode, and Shift is utilized to start the system in safe mode.