The argument concludes that there was no discrimination against women on the grounds that half of those admitted were women.
I would attack this on the grounds that half of the successful applicants being women does not prove a lack of discrimination. If 90 percent of applicants were women, but only half of those accepted were women, then there may indeed be discrimination.
This is a necessary assumption. We are being asked what the author must agree with. The most obvious thing they must agree with is that the applicant pool must not be mostly qualified women, but there could be other options.
That said, D completely fixes the flaw. If D were not true, and the majority of qualified applicants were women, then the authors argument would fall apart.