Select the Venn diagram that best represents the relationship between the…

Concept — In a class-relationship Venn question every named class is drawn as a circle, and the way any two circles are placed encodes the relation between those two classes. Only three placements exist: one circle drawn wholly inside another means every member of the first class is also a member of the second; two circles that cross mean some members are shared while each class also keeps members of its own; and two circles drawn apart mean the classes share no member at all.

Application — Test each pair among the three given classes — Fairies, Mythical creatures, Unicorns — against those three placements.

1. Every fairy is a mythical creature, and there are mythical creatures that are not fairies. So the Fairies circle must lie wholly inside the Mythical creatures circle.

2. Every unicorn is a mythical creature, and there are mythical creatures that are not unicorns. So the Unicorns circle must also lie wholly inside the Mythical creatures circle.

3. No fairy is a unicorn and no unicorn is a fairy — they are two separate kinds of mythical creature. So the Fairies circle and the Unicorns circle must be drawn apart, sharing no region.

Cross-check — Each of the other arrangements asserts something these three classes do not satisfy:

• Three circles that all cross one another would mean some fairies are unicorns, and that some fairies are not mythical creatures — both false.

• One circle inside a second circle with a third circle drawn completely apart would mean one of the three classes shares no member with the other two, yet fairies and unicorns are both mythical creatures.

• Three circles in a row where only neighbouring circles cross would mean that only some fairies and only some unicorns are mythical creatures, whereas all of them are.

Result — Putting the three placements together, the required diagram is one large circle standing for Mythical creatures that encloses two smaller circles standing for Fairies and Unicorns, with those two smaller circles kept apart from each other.