What key combination is used to forcibly halt the X server in the event of the graphical session becoming unusable (e.g., desktop manager or full screen application hanging)?

What key combination is used to forcibly halt the X server in the event of the graphical session
becoming unusable (e.g., desktop manager or full screen application hanging)?

A.
press CTRL+ALT+DEL

B.
press CTRL+ALT+BACK_SPACE

C.
press CTRL+ALT+DEL twice

D.
press CTRL+ALT+F1 then run