Here the question asked for binary tree
It can be of 2 types (1) skewed tree (2) Balanced binary tree or AVL tree
We have to find external path length i.e. leaf node
We also know cost of external path = leaf node value * lenth of path
Now for balanced tree external path length=$n\times log n$
But for skewed tree it will be $O\left ( n \right )$ only
So, ans will be D)