You want to terminate your X session.

The Ctrl+Alt+Backspace key combination is used to terminate the X session.

Answer option A is incorrect. The Ctrl+Alt+F7 key combination is used to switch from a virtual console back to the X terminal.

Answer option C is incorrect. The Ctrl+Alt+Fn key combination is used to switch from graphical mode to the nth (n varies from 1-6) virtual console running in text mode.

Answer option D is incorrect. Ctrl+Alt+A is not a valid key combination to terminate the X session.