Which command allows you to make a shell variable visible to subshells?

A.
export $VARIABLE

B.
export VARIABLE

C.
set $VARIABLE

D.
set VARIABLE

E.
env variable

Explanation:
From the man pages:
export [-fn] [name[=word]] …
export -p
The supplied names are marked for automatic export to the environment of subsequently executed commands. If the -f option is given, the names refer to functions. If no names are given, or if the -p option is supplied, a list of all names that are exported in this shell is printed. The -n option causes the export property to be removed from each name. If a variable name is followed by =word, the value of the variable is set to word. export returns an exit status of 0 unless an invalid option is encountered, one of the names is not a valid shell variable name, or -f is supplied with a name that is not a function.

You can not use export $VARIABLE, because the shell would expand $VARIABLE to its content before calling export.