It can reasonably be inferred from passage 1 that, for most bird
species, there is a certain size at which they

If Passage 1 is adapted from "Ancient DNA Tells Story of Giant Eagle  

Evolution," © 2005 by Public Library of Science, then the answer would be as follows:

It can reasonably be inferred from Passage 1 that, for most bird species, there is a certain size at which they  

a.are unable to fly.

Explanation:

According to Passage 1, "When the dodo's ancestor (thought to be a migratory pigeon) settled on this island with  abundant food, no competition from terrestrial mammals, and no predators, it could survive without flying, and thus was freed from the energetic and size constraints of flight. "

"Size constraints of flight" means that the birds that are big to a certain size cannot/could not fly. Taking examples of some birds once roaming New Zealand into consideration, they were giant birds with large wingspans, and therefore, they were unable to fly (flightless).

Other options, namely b. require less energy when flying; c.are more likely to attack other animals; and d.do have a to compete with other birds for food, are nowhere found in Passage 1. In other words, No related information is available.

Therefore, the most possible answer is a.