Which of the following commands allows an administrator to make a shell variable visible to subshells?

`export $VARIABLE` attempts to export the *value* of the variable as a new variable name, which is syntactically incorrect and not the way to make a variable visible to subshells.

The `export` command, when followed by a variable name without the dollar sign, marks that variable for export to the environment of subsequent child processes. This makes the variable accessible to any subshells launched from the current shell.

The `set` command is used to display or modify shell options and positional parameters, or to list shell variables, but it does not export variables to subshells.

Similar to choice C, `set VARIABLE` would just display the variable's value if it exists or define a positional parameter, and does not make the variable visible to subshells.

The `env` command displays the current environment or executes a command in a modified environment; `env VARIABLE` would attempt to run `VARIABLE` as a command with an empty environment, which is not the correct usage to export a variable.