Why not A

I think the logic of A is reversed, and indeed it think it's a mistaken reversal of the right answer. We are looking to prove that because reptiles are incapable of alteration they must be incapable of complex behavior. D directly supports/states this. A however tells us that the only way they could make alterations is if they were capable of complex behavior. Since we don't know if they are capable, it doesn't matter what is necessary for them to be capable.