Catalan Number gives the count of distinct binary trees with nodes that cannot be distinguished with each other,i.e unlabelled nodes and only the structure of tree is different.
But, here nodes are distinct A,B,C,D,E and total permutations of them = 5! = 120 .
So, each structure of tree includes these permutations.
Hence, maximum number of BT = Catalan number * 120 = 5040