Answer: People will let their guard down and not take precautions to prevent the spread of the Zika virus.
Explanation:
You did not include the declaration but this should be the correct answer as it is the best out of the rest.
Option B is wrong because people will still be infected with the Zika virus as it has not been completely defeated.
Option C is wrong as well because more money being spent on a Zika virus vaccine is a good thing so experts would not be worried about that.
Option D is also a positive thing so experts would not be worried about that as well.
Option A is the most feasible answer because the WHO most probably made a declaration that would make people relax their guard in relation to the Zika virus and this would be bad because it might lead to the virus spreading once more.