Why is D incorrect?

@Shiyi-Zhang, Happy to help. (D) says, "It presents a new finding that challenges the consensus view of the structure of pulsars." "New finding" is what allows us to get rid of (D); we do not have any sort of reason to believe that this is a brand new finding or discovery, so we can get rid of this choice. (A) says, "It helps explain how pulsars could have neutrons on their surface even if they were not entirely made up of neutrons." (A) helps us to clarify why it'd still make sense for a pulsar to be filled with quarks if we've observed them with a surface of neutrons. Thus, (A) is the correct answer choice. Does this make sense to you? Let us know if you have any other questions!