Hey Moshe,
A few points that might help out here. Always keep in mind that we don't try to explain things from our view, or your view, but rather from Law Services' view. They are the arbiters of what is right and wrong on this test, and understanding and explaining their viewpoint is always our goal. So, I think your request to explain it from your view probably isn't the right approach
That said, I think we can all agree that (B) strengthens the argument, and is the correct answer. Thus, the question comes down to the value of (E), and whether it strengthens the argument in any manner in the opinion of the test makers. I don't want to review all the prior comments on (E) in detail because I don't see the use in it, but in my opinion, (E) actually hurts the argument, if anything. If dolphins are exhibiting these same types of behaviors in their natural habitat, it may mean that the responses being exhibited to the hand signal are just random behaviors and not actually the product of higher cognitive functions being exhibited in response to the hand signal. In stating this view, it's my belief that the dolphins in their natural habitat have not been trained (that's my interpretation of "natural"). But, what if they had been trained and were just out there palling around in a non-trainer-present environment? Would that belief change this answer to something that strengthens the argument? In my opinion, no. If that was the case, I don't see it specifically helping the argument because we can't be sure what to make of that fact--is it a random behavior or the result of higher cognitive function?
Thanks!