Since it is a binary search tree, its inorder traversal produces a sorted sequence i.e. $\text{10, 15, 20, 23, 25, 30, 35, 39, 42}$.
Now given inorder and preorder traversals, we get following tree :
From this, we can give postorder traversal sequence as $\text{15,10,23,25,20,35,42,39,30}$ i.e. option (D).