Number of diagonals in a polygon \(={ }^{n} C_{2}-n=n\) \[\] \(\Rightarrow{ }^{n} C_{2}=2 n\) \[\] \(\Rightarrow \frac{n(n 1)}{2}=2 n\) or \(n=5\). Thus the polygon is a pentagon.\[\]Choice (a)\[\]