In this case, the correct answer is b. some sources say the flu vaccine could lessen negative effects on the economy.
Explanation:
The text explicitely mentions the impact of the flu vaccine could have to lessen the costs of medical treatments and also to deal with other indirect costs such as employees having to stay at home due to the sickness. It is explicitely stated the average cost of flue and its impact on the economy.
I believe it is option "D.) The flu vaccine is not reliable enough to be worthwhile" It would have a negative impact on the economy due to people not spending money to have the flu vaccine.