Okay, sis Hiba. Look my correction.
Here, you are trying to write a square root into fraction. To do that, you just have to apply the formula :
[tex]x ^\frac{power}{root} [/tex]
Then, here it gives [tex]8 ^\frac{x}{3} [/tex].
In short, the answer would be the option C.
Hope this helps !
Photon