Answer (D)
Below trees are possible if the tree has four nodes
These trees are for unlabeled nodes. Since we have a trees with labels, so each tree in the above diagram, can
be permuted with labels. Since there are four labels (A, B, C, D), so tree can be permuted 4! times.
So there are total of 5 trees possible and each of it can be permuted 4! times.
Therefore total trees possible would be
5 x 4! = 5! = 120