Which relationship can be determined from this dichotomous key?

The correct answers are : Organism Z is genetically closer to organism X than to organism Y and  Organism X and organism Z have at least one observable trait in common. 

Dichotomous keys are constructed by observing the evolutionary relationship between different species, genera etc.
The closer the two species are on the dichotomous tree/key are ( the closer to them is their common ancestor) the more closely related they are (they have more common traits)
As we can see in this key, species X  and Y are closely related and distant to species Y.