Thanks for the question! So first, realize that this is a strengthen with necessary premise question since it’s asking for an assumption required by the argument. What’s the conclusion here? “This isn’t true.” In other words, the conclusion is that the public isn’t afraid of scientists. And why? Because the scientist has never met anyone herself who’s afraid of scientists, and she’s been a scientist for decades.
OK, so now we need to find what the author has to be assuming if her argument is to be valid. Well, basically, the author is assuming that if the public were afraid of scientists, then someone in her position would have had to meet someone afraid of scientists by now. And that’s exactly what (D) says, making it the correct answer.
Hope this helps! Feel free to ask any other questions that you might have.