After some thought, I agree that this may be the way to go under time pressure, except that the minimum number is 7, not 5.My thoughts: the number of items bought have to lie between 5 and 12 (5 + 7).

So a good guess would be E.

A full solution is the following. Let us define the sets of isles: I1 = {1, 2, 3}, I2 = {4, 5, 6, 7}, I3 = {8, 9, 10}. Note that these sets are pairwise disjoint, I1 together with I2 is {1, ..., 7} and I2 together with I3 is {4, ..., 10}.

Variant I: 9 - 7 = 2 items bought in I1, 5 - 2 = 3 items in I2, 7 - 3 = 4 items in I3.

Variant II: 10 - 7 = 3 items in I1, 2 items in I2, 5 items in I3.

Variant III: 4 items in I1, 1 item in I2, 6 items in I3.