The correct answer is (D).
(D) Assumption (Sufficient)
Step 1: Identify the Question Type
An assumption that would allow the conclusion to be properly drawn is a Sufficient Assumption.
Step 2: Untangle the Stimulus
The author concludes that it will be necessary to vaccinate high—risk individuals every year for a different strain of the flu virus because the vaccination will only protect against the strain anticipated to be most prevalent that year.
Step 3: Make a Prediction
The author jumps from the evidence—that a vaccine for any particular year only protects against the most prevalent strain predicted for that year—to a conclusion that every single year a new vaccination for a different strain will be required. The author ignores the possibility that the same strain might dominate in multiple years and that a previous vaccination will continue to work when that strain shows up again. In order for the author's conclusion to make logical sense, that possibility can't happen. So the correct answer will indicate that repetition of a prevalent strain will not occur.
If you did not recognize that, then for each answer ask if this would guarantee that every year high—risk individuals will need a new vaccine for a new strain. A Sufficient Assumption answer, when added to the evidence, should guarantee that the conclusion is true.
Step 4: Evaluate the Answer Choices
(D) is correct. If every year there will be a new dominant strain of influenza, then the conclusion that every year a different vaccine will be required must be true.
(A) does not guarantee that a vaccine for a different strain of the virus will be needed every year. The number of high—risk individuals is irrelevant, because the conclusion says all of them (regardless of count) need an annual vaccine.
(B) is Out of Scope. The issue is whether the prevalent strain of the virus will vary year to year, not whether the seriousness of the epidemic will vary from year to year. Additionally, the stimulus doesn't mention anything about epidemics at all; it just says the government is trying to limit serious cases.
(C) is a Faulty Use of Detail. It restates the evidence, which already indicates that each year's vaccine will protect only against the one strain most likely to be prevalent. So, adding this choice to the evidence wouldn't affect anything at all.
(E) is Out of Scope. The number of the vaccinations' side effects does not have any effect on whether a different vaccination will be needed every year.
