This is a Must Be True, Except question, at least that's probably how this question would classified, but I think it's better to think of this one slightly different.
4 answer choices = must be true (and I think in this case it's best to the think of these as most strongly supported rather than absolutely, positively true)
1 answer choice = could be false (not supported)
It's definitely possible that all 5 answer choices could be true. We're just looking for the only answer choice that is not supported.
The statements fundamentally connect youthful play in mammals with complex movement, posture, social response and survival skills.
(A) connects youthful play with complex movement
(B) discusses nonmammalian animals and is irrelevant.
(C) connects youthful play with social response
(D) connects youthful play with survival skills
(E) connects youthful play with survival skills
Your reasoning for selecting answer choice (B) was perfectly valid. We were looking to extract information from the statements and use that information to discover which answer choice was least supported. Since the stimulus did not mention nonmammalian animals, answer choice (B) could not be supported by the stimulus.