Rightmost Element of BST= Largest Number
Left most Element of BST= Smallest Number
Rightmost of BST(B1)=5 and Leftmost of BST(B2)=1
Difference=5-1
=4,So D is the correct answer.
Binary Search tree(B1) with 1,2,3,4,5 & Binary Search Tree(B2) with 5,4,3,2,1
1 5
2 4
3 3
4 2
5 1