Correct Option: C
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 )