Simple way is, in question clearly say that Preorder is "abc" and Postorder is "cba", that means root element is 'a' and leaf element is 'c'.
So that, we can design 4 binary trees with the help of remaining element i.e., 'b'.
'b' can be left child of 'a' and/or right child, similalry 'c' can be left child of 'b' and/or right child.
total 4 combination.