$L =$ leaf nodes
$I =$ internal nodes
$T =$ total nodes $= L + I$
in a tree no. of edges $= T - 1$
all edges are produced by only internal nodes so
$k*I = T-1$ ....................(1) (for $k-ary$ tree, in this question $k = 3$)
$L + I = T$.....................(2)
solving $1$ and $2$ we get
$L = ((k-1)T+1)/k$
So put $k = 3, T = n$
you get $L = (2n+1)/3$
Ans D