Answer :-> C)n(k-1) +1
Originally when we have root , there is only 1 node, which is leaf.(There is no internal node.) From that "+1" part of formula comes from this base case.
When we k children to nodes, we make root internal. So then Total Leaves = n(k-1) + 1 = (k-1) + 1 = k
In k complete k ary tree every time you add k children , you add k-1 leaves.( +k for leaves, -1 for node which you are attaching )