The argument fails to consider the possibility that walking on hind legs is the result of both learning
Why this is right
Since this answer begins with fails to consider / overlooks the possibility, we can ask ourselves whether it would be an Objection. If we say that "walking on hind legs is both learned and innate", is that an objection? Sure! The conclusion said that "walking on hind legs is not learned", so this is basically contradicting part of the conclusion. That definitely weakens, so this counts as an objection. More conversationally -- the author was thinking that since the Kodiak bear has anatomical features that make it a great candidate for walking, its walking must be by anatomical design, not by learned behavior. And this answer is just saying, "couldn't it be that the anatomical underpinnings were there, but the Kodiak bear also required some learning in order to walk the way it does?" We could look at the bodies of Usain Bolt or Michael Phelps and say, "Their sprinting or swimming skills, respectively, are instinctive not learned." But the reality is that they're both -- they have body types that are particularly well suited to their sports, but they still had lots of training (i.e. learning) to optimize that capacity.
Skill tested: Flaw · how this choice captures the argument's function is the move to repeat next time.