Which of the following configuration files should be modified to set default shell variables for all users?

`/etc/bashrc` is processed for interactive non-login Bash shells, typically setting shell-specific functions and aliases, but `/etc/profile` is used for system-wide variables upon login.

The `/etc/profile` file is executed for all users when they log in to a Bourne-compatible shell (like Bash) and is the standard location for setting system-wide environment variables and commands. This ensures that all users inherit these common settings upon starting their login session.

`~default/.bash_profile` is a per-user configuration file for a specific user named 'default', not a system-wide setting for all users.

`/etc/skel/.bashrc` provides template files that are copied to a *new* user's home directory during account creation, not a file applied directly to all existing or new users.

`/etc/skel/.bash_profile` provides template files that are copied to a *new* user's home directory during account creation, not a file applied directly to all existing or new users.