I think I got it:
First of all, what will the correct answer accomplish?
Which one of the following would it be most helpful to know in order to judge whether what the scientist subsequently learned calls into question the hypothesis?
In other words, the correct answer would provide the info on why the second observation questions the scientist's hypothesis; i.e., the second observation will weaken his hypothesis in light of the new information provided by the correct answer.
(B) whether the types of foods that migratory fish eat while they inhabit the ocean are similar to those that they eat while they inhabit bodies of fresh water
If we answer 'yes' to this question, will the second observation weaken the hypothesis?
No. If the food is similar everywhere, then it doesn't refute the hypothesis that fish travel in accordance with the availability of food.
If we answer 'no' to this question, will the second observation weaken the hypothesis?
No. Imagine that in the ocean fish eat mollusks type A, and in fresh water they eat mollusks type B. If for the growing phase of their life they need mollusks type A, and these mollusks are available in the ocean in temperate zones and in fresh water in tropics. Fish behavior in tropics is again explained by the hypothesis, and the hypothesis is not refuted by this info.
Therefore, knowing the information from (B), will not help the second observation to refute the hypothesis.