The best answer is B. stimuli discrimination.
Thor's failure to fetch the ball when Glen yells "Pineapple!" indicates that he is unable to discriminate between the specific command that he has been trained to respond to and the new term that Glen used. In operant conditioning, discrimination occurs when an animal learns to respond differently to different stimuli. In this case, Thor has not learned to associate "Pineapple!" with the action of fetching, demonstrating that he can differentiate between commands he has learned and those he has not.