Yasukawa's study of blackbirds showed that the percentage of smaller birds that survived the duration of the study exceeded the percentage of larger birds that survived. The criticism states that Yasukawa's conclusion that size is a determinant of a blackbird's chances of survival over a month-long period is probably mistaken, because smaller blackbirds are generally younger than larger ones.
If Yasukawa was studying birds from only one species of blackbird, then the fact that the smaller blackbirds are generally younger than larger ones weakens the conclusion that size is a determinant of a blackbird's chances of survival because it adds an added factor of youth into the equation, i.e. the younger birds, which happen to be smaller, have a higher chance of survival over the month-long period than the older birds, which happen to be larger.
However, if Yasukawa was studying two different species of blackbirds, one that, as a species, is larger and one that, as a species, is smaller, then the added factor of age will be cancelled out and the only differing variable will be size thereby making it the determinant of survival over a month-long period.
So, if answer choice (A) "Yasukawa compared the survival chances of two different species of blackbirds, a larger and a small species, rather than of different sizes of birds within one species," were true, then the criticism of Yasukawa's research is based on a misunderstanding. Thus, answer choice (A) is the correct answer.
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