Next, London develops the theme of instinct. “The domesticated generations fell from him” means that Buck is unlearning all of the behavior that centuries of breeding have instilled in dogs. “In vague ways he remembered back to the youth of the breed” means that he has a memory deep inside of what it meant to be a beast thousands and thousands of years ago, when dogs were a new, young breed, forced to fight and kill to survive
The correct answer is D) The racial unconscious