$\begin{array}{1 1}(A)\;\large\frac{n+1}{2}\\(B)\;\large\frac{n+2}{2}\\(C)\;(\large\frac{n+1}{2})(\large\frac{n+2}{2})\\(D)\;\text{None of these}\end{array} $

And the answer of the question is (n+1)(n+2)/2