OFFICIAL EXPLANATION
(A) The percentage of allergy victims who were not aware of the allergy before a major episode
This answer makes an Irrelevant Distinction. Knowing the exact percentage doesn't actually tell us anything
(B) The percentage of the population that is at risk for allergic reactions
This answer is Out O f Scope because it talks about all allergies in general, not just food allergies.
(C) Whether some of the eight foods are common ingredients used in cooking
This doesn't fit into one o f the standard trap categories. The argument does not hinge on how commonly used the foods must be in order to warrant testing. Further, the argument does not limit itself to foods that must be cooked.
(D) Whether an allergy to one type of food makes someone more likely to be allergic to other types of food
This answer makes an Irrelevant Distinction; the argument doesn't address whether someone is allergic to multiple types of food.
(E) Whether ingesting a very small amount of an allergen is sufficient to provoke an allergic reaction in a susceptible individual
This is the correct answer.