Answer choice (D) is not a biconditional statement like you find with "if and only if" or "if but only if." Instead, it's a type of rule that we typically refer to as the "at least one" rule.
Consider this example:
If not A, then B.
This relationship tells us that you always have to have at least one of A or B. If A isn't there, B has to be. If B isn't there, A has to be. So there's no way to have neither A nor B. That means we always have to have at least one of A and B. We could have both A and B. But we can't have neither. When you're looking at a contrapositive pair like we have above, you can't have the two sufficient conditions at the same time (in this case, we can't have not A and not B at the same time) but you could have the two necessary conditions at the same time (it would be fine to have both A and B).
So applying that to answer choice (D):
Unless Hibiscus is second, it must be somewhere between Magnolia and Gardenias.
Contrapositive: M/GK H GK/M
(Diagramming note: You can absolutely split up the sufficient condition in the contrapositive as you did, if it makes it easier for you to understand. Your diagram is fine and whichever way of diagramming makes it easier for you to understand the relationship correctly, is the right way to diagram for you!)
Based on our analysis of "If not A, then B" above, we can see that this relationship works in the same way: if H is not 2nd, H is between M and GK. If H is not between M and GK, then H is 2nd. That means one of those two things always has to happen. We always have to have either H 2nd or H between M and GK.
Hope this helps!