I also have the same question as the person above, why D over C?

@Patricia-Adekunie, This is a point of issue question, meaning we are looking for an issue that two speakers disagree about. Let's briefly look at the dialogue: W cites the evidence that researchers agree there is no "manic-depression gene" in support of the conclusion that any claim that some people are genetically predisposed to manic-depression is false, i.e W is saying no one is genetically predisposed. C accepts W's evidence but offers an alternative explanation for genetic predisposition, i.e. interaction among several gens, for manic-depression to question W's conclusion. Let's look at (C) & (D). (C) is incorrect because W cites the evidence that "nearly all researchers now agree that there is no "manic-depression" gene and C clearly agrees with this evidence - "I do not dispute your evidence." Both C and W agree that there is no single gene responsible for manic-depression, but C argues that a combination of certain genes might make one genetically predisposed. (D) is correct because it accurately describes the point of disagreement between C & D. (D) talks about people rather than genes, saying "current research supports the claim that no one is genetically predisposed." This is the paraphrase of W's conclusion. C refutes the claim that NO ONE is genetically predisposed citing additional evidence that "many researchers found a set of gens," suggesting that it is likely that at least SOME people could be genetically predisposed to manic-depression. Does this make sense? Let me know if you have any further questions.