Being an only child has little to do with a child's social development.

Ok , so far so good. The only child can have a normal social development


A recent study that followed thirty only children and thirty-five first-born children to the age of three found that the two groups of children behaved very similarly to each other toward their peers, their parents, and other adults.

To back up our claim we do have a comparison between the only child and first-born children. However, we do not know if the first born-children then have siblings

Which of the following, if true, most weakens the conclusion drawn above?

(A) The groups being compared did not contain the same number of children.

To find out if the two groups behave in the same fashing we already do know that they are different in number. Cross of. No new information

(B) More time was spent observing the interactions of children with their mothers than with their fathers.

Irrelevant

(C) Most of the researchers involved in the study were persons who had no brothers or sisters.

The family configuration of the researchers does not care for us. We care about the child

(D) The first-born children were, on the average, nearly three when their parents had second children

Correct - the first-born children could behave differently because after three years they have had a sibling. Having a brother or a sister help a lot to behave normally. Also, this confirms the stem: thirty-five first-born children to the age of three. And does not contradict it. I.E the comparison between the two groups is faltered because the first-born had a sibling and we could have a different scenario with the only children

(E) The "other adults" described in the study consisted mainly of members of the research team.

Other adults is irrelevant

Regards