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Answer: 8
Each fruit type needs to touch all of the other
three types, but each fruit can only be next to
two other fruits.
So there must be at least 2 fruits of every type $\Rightarrow$ at least 8 fruits altogether.
Find an arrangement with 8 fruits to show that this is possible:
OABPOBAP (O for Orange, P for Peach, A for Apple, B for Banana).