Not necessary, because this reverses the link from the premise to the conclusion. The author assumes that if reptiles are capable of complex reasoning, then they’d be able to make major changes in their behavior. (A) gets this relationship backwards.

Not necessary, because the author doesn’t propose any explanation for “all reptile behaviors.” Even if single stimulus-response explanations don’t account for all reptile behavior, they can still lack complex reasoning on the basis of their inability to make major changes in behavior.

The author never relies on a comparison of behavior between the field and the lab. So the author doesn’t have to assume anything about how reptile behavior compares in the two contexts.

Necessary, because this is the link the author uses to get from the premise to the conclusion. If this were not true — if reptiles could be capable of complex reasoning even if they NEVER make major alterations in their behavior — then the fact reptiles can’t make major alterations would not imply a lack of complex reasoning.

Not necessary, because even if complex reasoning and stimuli response can contribute to the same behavior, we still know that reptiles can’t make major alterations to their behavior. The author can believe that this implies lack of complex reasoning, even if in theory complex reasoning and responses to stimuli are not mutually exclusive. The author didn’t argue, “Because reptiles’ behavior is due to reponses to stimuli, it’s not due to complex reasoning.”