Which key combination is used to start up a Mac in Single User mode?

Starting a Mac in Single User mode is done using the Command-S key combination. Single User mode boots the system into a minimal environment suitable for troubleshooting, allowing access to a command-line interface where various system and disk utilities can be utilized. This mode is particularly useful for repairing filesystems, resetting passwords, or diagnosing problems when the standard graphical user interface is not accessible or functional.

The other options involve different startup modes: Command-V is used for Verbose mode, which provides real-time feedback during the startup process, showing system messages on the screen. The Shift key is for Safe mode, which loads only necessary extensions and disables startup items to help troubleshoot issues related to system performance or stability. The Option key allows users to select from available startup disks. Each of these modes serves distinct purposes, but for entering Single User mode specifically, the Command-S combination is the correct choice.