Answer:
D. - By showing how vegetarian diets can do serious harm to animals.
Step-by-step explanation:
You can tell that the author is talking about how dangerous that is because of the various pieces of evidence. "Cats are natural carnivores, which means they only eat meat. By changing this necessary part of the cat's diet, the couple
caused serious health problems for the animal." That theme is littered along the text.
Hope that helped! :)