Hi, the argument is flawed since it compares two groups of people, and decides that since the group with pets is generally less happy, the pets must be causing them to be less happy. It ignores the possibility that pets make people happier than they would have been in the first place. This is exactly what D is saying. If a pet makes people happier, then the argument fall apart. And, the author never actually proved that pets do not make people happier. If d were true, that would enable us to say that people who are sad in the first place get pets, which is one possible alternative explanation for the facts the author cites.
