What is the quick key combination to reboot a machine?

The key combination that is commonly utilized to reboot a machine is indeed CTRL+Alt+Delete. This combination is recognized for its function in various operating systems, particularly in Windows, as it brings up the security screen that allows users to log out, switch users, or access the task manager, from which they can restart the computer.

The other choices provided do not serve the same purpose for rebooting a machine. The use of ALT+F4 is generally for closing applications rather than rebooting. CTRL+Shift+Esc opens the task manager directly, which does not lead to a reboot by default. Similarly, the last pair of combinations like ESC+F5 or ALT+F6 do not have recognized functions associated with rebooting a machine and are not standard shortcuts across widely used operating systems. Therefore, the first choice stands out as the correct answer due to its established functionality in initiating the reboot process.