The listener HttpSessionBindingListener is for the object to know when it has been added (bound) or will be removed (unbound) in a session.
If you want to know when anything has been added, replaced or removed as an attribute in a session, you have to use HttpSessionAttributeListener. Pay attention: HttpSessionAttributeListener must be declared in the DD.
In you example, the Dog implements HttpSessionBindingListener and it is notified when it is added or removed in a session.
What mock exam do you use. I saw similar question in EnthuWare mock exam, and people on their forum says that HttpSessionBindingListener is a correct answer for such questions. We can only hope not to encounter such questions in the real exam