Which command makes the shell variable named VARIABLE visible to subshells?

`export $VARIABLE` is syntactically incorrect for exporting a variable; it attempts to use the *value* of the variable as the name of the variable to be exported, which will fail or produce an unintended result.

The `export VARIABLE` command marks the specified shell variable to be placed in the environment of any child processes that are subsequently created from the current shell, thus making it visible to subshells.

`set $VARIABLE` sets the shell's positional parameters to the words contained in the variable's value and does not export variables to subshells.

`set VARIABLE` simply displays shell variables or sets shell options, it does not export variables to subshells.

`env VARIABLE` (if VARIABLE is a value) or `env -i VARIABLE=VALUE command` runs a command with a modified environment or displays environment variables, but it does not export the variable from the current shell for general subshell inheritance.