Now A[2] have d children from A[(d+2)] to A[(d+2)+d]

A[3] have d children from A[(d+2)+d+1] to A[(d+2)+d+d]..

now observe, we are adding d to each node from A[2] to A[d+1] i. e. total d nodes..

So, if last children of node A[2] have index A[(d+2)+d],

then last children of node A[3] have index A[(d+2)+2d]

Similary last children of A[d+1] will have index A[(d+2) + d*d]

because there are d nodes from A[2] to A[d+1] and each has d children, so we are adding d nodes d times...

To solve this question, take some small values of d and check the options..