Not necessary, because whether we can “understand” why people want to avoid manipulation has nothing to do with we can infer the public’s lack of fear of scientists from the author having never met someone afraid of scientists.

Not necessary, because we care about whether the fact the author hasn’t met someone who’s afraid of scientists proves that the public isn’t afraid of scientists. Whether someone “understood what science is really about” has nothing to do with the reasoning.

Not necessary, because the argument concerns whether the public is afraid of scientists. Whether they are afraid of tech development that results from science is a separate issue.

Necessary, because if this were not true — if over several decades a scientists might NOT encounter someone who was afraid of scientists, even if the public actually is afraid of them — then the fact the author hasn’t met someone afraid of scientists would not prove that the public doesn’t fear scientists. (D) is the necessary link to get from the premises to the conclusion.

Not necessary, because the argument doesn’t involve people who have claimed they are afraid of scientists. (The first line tells us some pundits say the public is afraid of scientists, but this is not an example of people saying that they are afraid of scientists.)