Ans is (d)
No. of unlabelled BSTs with 'n' nodes= C(2n,n) / (n+1)
With n=3, we get 5 unlabelled BSTs or 5 BST structures.
Each structure has 3 nodes which can be arranged in 3! ways. Out of these 3! ways for each BST, only one order corresponds to the given post-order. Thus, each of the 5 BST structure has only one way in which the given post-order can be achieved.
Therefore, answer is 5.