https://toefl.gregmat.com/quizzes/quiz/listening-exercise-symbiosis (Question 4)
Option D can also be the answer.
The lecture was to talk about the 3 types of symbiosis. In that regard, the example was to illustrate commensalism where one species benefits while the other is unaffected. Hence, saying ‘‘whale is neither helped nor harmed’’ is to demonstrate how the whale is unaffected in the symbiotic relationship. The Prof therefore did not give the example to show that whales do not need barnacles.