We have been given the expression 
 .
. 
To solve this problem we will use one of the rules of the exponents called the Power Rule.
The power rule states that:

In our case, x=a and the powers involved are m=8 and n=4.
Therefore, applying the power rule to the case given in our question we get:
 .
.
Therefore, Option D which says 
 is the correct option.
 is the correct option.