Given for every $n \in N \;(1^2-a_{1})+(1^2-a_{2})+.......+(n^2-a_{n})=\large\frac{1}{3}n(n^2-1)\;the\;a_{n}$ equals

Question

$(a)\;n\qquad(b)\;n-1\qquad(c)\;n+1\qquad(d)\;2n$