$L =$ leaf nodes
$I =$ internal nodes
$n =$ total nodes $= L + I$
In a tree no. of edges $= n - 1$
All edges are produced by only internal nodes so,
$k\times I = n-1\qquad \to(1)$ (for $k-ary$ tree, in this question $k = 3$)
$L + I = n\qquad \to (2)$
Here, given options are in terms of "n". So, eliminating $I$ from $(1)$ and $(2)$,
$L = ((k-1)n+1)/k$
you get $L = (2n+1)/3$
Answer is D.