(A) separates the duty to protect an individual animal from the duty to protect that animal’s species. This undermines the argument, because we now have less reason to think that we have greater duty to protect a whooping crane than a sandhill crane.

(B) builds a bridge from the premise to the conclusion. Since an individual whooping crane is more important to preservation of the whooping crane species than an individual sandhill crane is to the preservation of the sandhill crane species, (B) establishes that we have a greater duty to protect the lives of individual whooping cranes than we do individual sandhill cranes.

The concept of how “closely related” one species is to another has nothing to do with the reasoning of the argument.

The argument doesn’t involve a comparison between the importance of protecting the species to the importance of protecting individuals of that species.

(E) establishes what’s necessary (”only if”) in order for there to be a greater duty to protect one kind of animal over another. But that doesn’t help us establish that we do have a greater duty to protect one kind of animal than another. (If you think it does, you’re confusing sufficient and necessary conditions. “X only if Y” NEVER allows us prove that X is true. So “Greater duty only if Y” NEVER allows us to prove that we have a greater duty.)