Smaller adult iguanas are found on Plazos Island compared to nearby islands. Researchers assumed that Plazos Island supports smaller hatchlings. But they found some evidence that smaller hatchlings are at greater risk from predators, compared to larger hatchlings.
So we need some different evidence to explain how smaller iguanas are found in Plazos Island. One possibility could be some condition that threatens the larger iguanas but doesn't affect the smaller ones.

(A) Periodic wind shifts cause extended dry spells on Plazos every year, putting the larger iguanas, whose bodies require relatively more water, at a great disadvantage.
So, larger iguanas are at a disadvantage here and could be the reason for finding small iguanas
(B) There are exactly three species of iguanas on Plazos but only two species of seagulls that feed on iguanas, and a relatively small percentage of each year's hatchlings are consumed by seagulls.
This connects with the evidence found in the passage, that the smaller hatchlings are at a greater risk as they cannot easily evade predators. So, it is opposite of what we want.
(C) Wild cats, which were introduced as pets by early settlers and which were formerly major predators of Plazos iguanas, were recently killed off by a disease specific to cats.
If this was true, it would mean that both small and large iguanas should have been found on Plazos Island. But that's not the case.
(D) The iguanas on Plazos are a relatively ancient part of the island's animal life.
Out of context.
(E) Both land and marine iguanas live on Plazos, and the land iguanas tend to be larger than marine iguanas of the same age.
Also out of context.

Hence, option (A) is correct.