$\begin {array} {1 1} (A)\;\large\frac{n^2+1}{12} & \quad (B)\;\large\frac{n^2-1}{12} \\ (C)\;\large\frac{(n+1)(2n+1)}{6} & \quad (D)\;\bigg[ \large\frac{n(n+1)}{2} \bigg]^2 \end {array}$
