This is a sufficient assumption question, one in which we are given the conclusion in the question. We need the answer choice to provide a bridge between the support we are given and the conclusion.
Conclusion: number of adolescent boys with asthma is equal to number the number of adolescent girls with asthma
Support: the percentage of adolescent boys with asthma is about the same as the percentage of adolescents girls with asthma.
The stimulus gives us alot of background information, but we want to focus on the part that relates to adolescents because that is what we are dealing with in the conclusion.
The issue here is that the author jumps from talking about percentages to talking about whole numbers. C solves that issue for us by telling us that there is an equal number of adolescent boys and girls. By knowing that, and using our info about equal percentages, we can conclude that the number of adolescent boys with asthma is equal to the number of adolescent girls with asthma.