Happy to help. We're looking for the answer choice that's most strongly supported by the stimulus. Let's take a look at (E).
(E) says, "The evolutionary significance of Tiktaalik could not be determined just through comparison to fish species of its time."
(E) is precisely what we're looking for. The stimulus tells us that when the Tiktaalik was around, it didn't really stand out because there was so much variation between fish. Other fish had unique characteristics, too, so only over a long period of time and within the context of animal evolution as a whole could we realize just how important the Tiktaalik's fingers were, and (E) picks up on this. Thus, it's the correct answer choice.
Hope this helps. Let us know if you have any other questions!