The biologists hypothesize that young harbor seals start with an aversion to all killer whales and then learn to ignore those who do not eat seals.
Answer choice (E) is incorrect because it does not demonstrate that young harbor seals start off with an aversion to all killer whales. It does support that harbor seals will ignore fish-eating killer whales, but we are not provided with any evidence to support the hypothesis that this is a behaviour which is learned over time.
Answer choice (C) may not be about young harbor seals, but it still supports the hypothesis. Based on the hypothesis, mature harbor seals would have feared all killer whales in their youth, but then learned to ignore fish-eating killer whales. However, if they were unfamiliar with a fish-eating whale's dialect, this would mean that they would not have learned to ignore it and would still hold on to the aversion from their youth.
Hope this is helpful! Please let us know if you have any further questions.