$\begin {array} {1 1} (A)\;\large\frac{n+1}{2} & \quad (B)\;\large\frac{3n(n+1)}{2(2n+1)} \\ (C)\;\large\frac{(n+1)(2n+1)}{6} & \quad (D)\;\large\frac{n(n+1)}{2} \end {array}$