Which of the following is the BEST way to list all defined shell variables?

`env` lists only the environment variables, which are a subset of all defined shell variables; it does not show local shell variables or functions.

The `set` command, when executed without any arguments, displays all shell variables (local and environment), shell functions, and shell options that are currently set in the shell's environment. This provides the most complete overview of the shell's state.

`env -a` is not a standard or valid option for the `env` command; `env` typically doesn't have a `-a` option for listing all variables.

`echo $ENV` displays the value of the `ENV` variable (if it's set), which typically points to a startup script, not a listing of all variables.