Correct Option: A
$x(n-1) +1$
Originally when we have root , there is only $1$ node, which is leaf. (There is no internal node.) From this base case "+1" part of formula comes.
When we $n$ children to root, we make root internal. So then Total Leaves $= \ = 1(n-1) + 1 = n$.
In complete $n$ ary tree every time you add $n$ children to node, you add $n-1$ leaves & make that node to which you are inserting childen internal.( $+n$ for leaves, $-1$ for node which you are attaching ). So if you had originally few leaves, you add $n-1$ "New" leaves to them. This is how $x(n-1) +1$ makes sense.