The no. of diagonals of $n$ sided polygon =?

Question

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