Its mentioned that node D will always be leaf node and tree has always 1 leaf node also. Hence we are left with 3 nodes.
At 1st level we can have 3C1 choices to select node
At 2nd level we have 2 nodes left so we can select 2C1 *2 as either we can put the node at right or left
at 3rd we have 1 node left and we can place it at either left or right so 2
at left D can be also placed at right or left so 2
so 3C1 * 2C1 *2 *2 *2 = 48 BT