The A.M of squares of first $n$ natural numbers is

Question

$\begin {array} {1 1} (A)\;\large\frac{n+1}{6} & \quad (B)\;\large\frac{n^2-1}{6} \\ (C)\;\large\frac{(n+1)(2n+1)}{6} & \quad (D)\;None\: of \: these \end {array}$