Let us do it like this :
No of nodes at depth 0 = 1 [ Root node ]
From this we have k children so
No of nodes at depth 1 = k
From each of this we will have k children in next level..
No of nodes at depth 2 = k2
Hence at depth k , no of nodes which will be leaf nodes as it is the
maximum level according to the question = kd
Hence C) is the correct answer
X->YZ , Y->XZ , ...