The conclusion of this argument is that humans would not be able to find aliens within 50 light years of us. Answer choice D is necessary for this because if it was negated, it would say that the other alien civilization that found us wouldn't want to contact us. If this was the case, then it is possible that humans would in fact be able to find an alien civilization within 50 light years of us because that civilization may exist but just didn't reach out to humans.
I hope this helps. Please let me know if you have any other questions!
