Conclusion:
"experts conclude that among members of the general
population who have spent a similarly large amount of time in close contact
with animals, the percentage with animal-induced allergies is not 30 percent
but substantially more."

In other words, a higher percentage of the general population will have animal-induced allergies. We need to strengthen this conclusion be selecting the answer that provide a reason why more people in the general population would have these animal-induced allergies.

A. This statement tell us that once people develop these allergies in the Zoo profession they switch to the general population. So the animal exposed general population should be at 30% plus the additional ex-Zoo workers which should make it higher. Correct

B. This is irrelevant because we are not concerned about the general population as a whole but instead only those in the general population who are exposed as much as Zoo workers. Even if it was relevant it would provide evidence that Zoo workers should have a higher rate of allergies and would weaken not strengthen.

C. This is irrelevant as well because were dealing with percentages.

D. As in B this would provide evidence that Zoo workers should be more exposed and weakens not strengthens.

E. This would also provide evidence to weaken and not strengthen.