since, A[1] is root node and its total d children are from A[2] to A[d+1]..
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..