We can solve this question by assuming some values like
Suppose we have 1,2,3,4 nodes it means n =4
1- If right subtree have 0 elements so p=0 put this value in option C which is n-p = 4-0 = 4
so we have to insert 1st element is 4 in this case BST is left skew
2- If right subtree have 1 element so p=1 put value in option C so n-p= 3
so 1st element is 3 in this case 4 is in right remaining in left
3- If right subtree have 2 element so p=2 put value in option C so n-p= 2
so 1st element is 2 in this case 3 and 4 will be in right remaining 1 in left
4- If right subtree have 3 element so p=3 put value in option C so n-p= 1
so 1st element is 1 in this case 2,3,4 is in right it is right skew tree
we can verify by this way in remaining options too but the correct option is C