The S.D of first $n$ naturals is

Question

$\begin {array} {1 1} (A)\;\large\frac{n+1}{2} & \quad (B)\;\sqrt{\large\frac{n / n+1}{2}} \\ (C)\;\sqrt {\large\frac{n^2-1}{12}} & \quad (D)\;None \: of \: these \end {array}$