Which of these commands will reboot the computer ?

Some keyboard combinations are reserved by the operating system to perform special control actions rather than typing characters. One of the most widely known of these on Windows-based systems is the combination that opens the Security or Task Manager screen, from which a user can choose to restart the computer, and on many older or unresponsive systems pressing it triggers a direct reboot.

Pressing Delete together with only one modifier key, or Ctrl and Alt without Delete, does not trigger this system-level interrupt. It is specifically the three-key combination of Ctrl, Alt, and Delete together that the operating system recognises as the reboot or system-interrupt command, because it was deliberately chosen as a combination unlikely to be pressed by accident during normal typing.

Remember this shortcut as a three-key combination; leaving out any one of the three keys changes its effect completely.