Thanks for the question! The problem with (D) is that it only justifies part of the reasoning in the argument. We can see how (D) would strengthen the idea that people won’t say anything bad about public figures in countries with strong libel laws. But (D) doesn’t tell us anything about why public figures in countries can’t have good reputations in countries with strong libel laws, and this is the crux of the question that we need to figure out.
Hope this helps! Feel free to ask any other questions that you might have.