An administrator has just set up the X Display Manager as the default display manager. Which of the following files should be edited to change the default background for it?

The `/etc/X11/xdm/Xsetup` script is executed by XDM before the login screen appears and is the designated place for applying session-specific configurations, including setting the desktop background.

`/etc/X11/prefdm` is usually a script or symbolic link determining which display manager to run, not a configuration file for the background of a specific display manager.

`/etc/X11/xorg.conf` is used for configuring the X server's hardware settings (e.g., monitor, input devices), not the visual elements of the display manager's login screen.

`/etc/X11/xdm.conf` (or `xdm-config`) is the main configuration file for XDM, but it typically delegates visual customizations like backgrounds to scripts such as `Xsetup` rather than defining them directly.