All leftward descendants of x’s right child r have keys between x and r; finding the least of these gives the successor of x.
The other thing is that x might not have a right child.
In that case, x is the rightmost descendant of some ancestor z, which is the left child of a node y (or z is the root, in which case x has no successor in T ).
Specifically, all rightward descendants of y’s left child z, including x, have keys between z and y. So y is the successor to x.
which supports option D .