If 3^n = 729 , then the value of n is ?

Answer: Option D

$$\begin{array}{rl}& \iff \sqrt{3^n}=729\\ & \iff \sqrt{3^n}=3^6\\ & \iff {\left(\sqrt{3^n}\right)}^2={\left(3^6\right)}^2\\ & \iff 3^n=3^{12}\\ & \iff n=12\end{array}$$