The $n^{th}$ term of series is $\frac{1}{1}$, $\frac{(1+2)}{2}$, $\frac{(1+2+3)}{3}$......is

Question

$(a)\;\frac{n+1}{2}\qquad(b)\;\frac{n-1}{2}\qquad(c)\;\frac{n^2+1}{2}\qquad(d)\;\frac{n^2-1}{2}$