The efficiency for recursively calculating $\backslash ($ x$^$n $\backslash )$ is $\_\_\_\_\_\_\_\_\_.$

Question $8$ options:

$\backslash ($ O$(\backslash$log$\_2$n$)\backslash )$

$\backslash ($ O$($n$)\backslash )$

$\backslash ($ O$($n$^2)\backslash )$

$\backslash ($ O$($n$\backslash$log$\_2$n$)\backslash )$