Which of the following is a "function expression"

a$.$ function add$($x$,$ y$)\{$

return x $+$ y;

$\}$

b$.$ var result $=$ add$(2,3)$;

c$.$ var result $=$ function$($x$,$ y$)\{$

return x $+$ y;

$\}$

d$.($function $($x$,$ y$)\{$

return x $+$ y;

$\})(2,3)$;