Binary trees can serve many different purposes. They don't necessarily need to be in an ordering like such.
Note that below, I hid key parts of the solution. You can view them by hovering over with your mouse, but I encourage you to try to solve it using clues from my response without seeing the actual answer.
For your particular problem, it appears that the ordering of the tree is such that all nodes to the 'right' have a value greater than our root (global or local root, doesn't matter) and all nodes to the 'left' have a value less than our root.
With this in mind, $u$ must be greater than our root node, so
is ruled out. Then, by taking $u$ to be our next (local) root node, $u$ must be greater than the maximum value of its left subtree and $u$ must be less than the minimum value of its right subtree. From this, we can see that
$28 < u < 50$.
This rules out
$24, 51, 58$.
Therefore, the answer to your problem is
$36, 45, 49$