Geometric mean of $1, 2, 2^2, 2^3.....2^n$ is

Question

$\begin {array} {1 1} (A)\;2^{\large\frac{2}{n}} & \quad (B)\;2^{\large\frac{n}{2}} \\ (C)\;2^{\large\frac{n-1}{2}} & \quad (D)\;2^{\large\frac{n+1}{2}} \end {array}$