If `D_k=1nn2k n^2+n+1n^2+n2k-1n^2n^2+n+1a n dsum_(k=1)^n D_k=56.` then `n` equals `4` b. `6` c. `8` d. none of these

Explanation

`sum_{k=1}^{n} D_{k}=[
[sum_{k=1}^{1}1, n , n],
[sum_{n=1}^{n} 2 k , n^{2}+n+1 , n^{2}+n ]
,[2 sum_{n=1}^{n}k-n, n^{2} , n^{2}+n+1]]`


`56=
[[n, n, n] ,
[n(n+1) , n^{2}+n+1 , n^{2}+n],
...