The best response to the counterargument is option D “Zoos may protect animals from being killed in the wild, but their lives are not much better if they're suffering.” since the focus of the counterargument is that in the zoo, animals are safer. The best response is to show that even if they are safer, animals' lives are not better if they are locked in a zoo and they are not happy there. Difference between living safer and living better, even if it implies facing the dangers in the wild.
The other options focus on the improvements the zoo has to make to give animals a better life in the zoo.