Conclusion: Baby-talk does not provide extra help to children in learning a language.
Support: In families that do not use baby-talk, children master the grammatical structure of their language just as well as children who are exposed to baby-talk.
This question leads me to ask, "Are there important aspects of learning a language other than the grammatical structure?" If so, perhaps baby-talk does help children in those aspects. We cannot conclude that it offers no extra help. Do you see how this calls the conclusion into question?
The argument depends on an assumption that fixes this hole in the argument. Answer choice C does so, because it shows that a mastery of grammatical structure is a sign of learning the language as a whole. A child cannot master the grammatical structure without learning the language. Since children in both groups do so just as well and just as quickly, we can accept the conclusion that baby-talk does not provide extra help.