The problem with D is that it is an extremely broad statement, and it fails to accurately represent the author's argument. First, the author is not making a claim about all behavior having only two possible explanations. He/she is only discussing Kodiak bears' walking ability. Second, the author does not allow for two possibilities, or "both of only two ways." The author narrows it down to one possibility, saying that the behavior is either instinctive or learned, but not both.
This leads us to the correct answer. Why can't something be both natural and learned? This criticism is given by B.