[ ( 81 )^2 ]^2 = ( ? )^2

Answer: Option E

$$\begin{array}{rl}& \text{Let,}\\ & {\left[{\left(\sqrt{81}\right)}^2\right]}^2={\left(x\right)}^2\\ & \text{Then,}\\ & \iff x^2={\left(81\right)}^2\\ & \iff x=81\end{array}$$