Answer: C
BST exhibit below properties:
1. Every element in the left should be less than or equal to the root.
2. Every element on the right side is greater than or equal to the root.
3. It has to be a binary tree.
else if(( root ---> left != NULL && root ---> data < root ---> left---> data)
||(root-->right != NULL && root ---> data > root ----> right ----> data))
This condition checks if the root's left is null or not. If it's not Null then check if the data of left is less than the root or not.
If its less than the root then recursively check the further nodes.
Similarly, the right node is being checked.