This question is asking you to find the answer choice which will make the argument "more persuasive." Essentially, this means you're looking for an answer choice which provides another premise in support of the conclusion. Here's a breakdown of the argument:
Premise 1: Defendants with private lawyers have a lower conviction rate than those with public defenders. Premise 2: ?? Conclusion: This is why criminals who commit crimes like embezzlement/insider trading are better at avoiding conviction than other criminals.
Answer D is correct because it helps to fill in a gap between the premise and conclusion. Premise 1 just tells us that private lawyers get their clients lower conviction rates than public defenders. This could be for a lot of different reasons-the crimes could be harder to prove or they could just be more likely to have not guilty clients. The answer choice provides rules out this second possibility (that they are just less frequently guilty). This makes the conclusion more likely, as another explanation for the situation has been ruled out.
