Final answer:
The dog's issue lies with stimulus discrimination, as it has learned to respond only to Mrs. Ridcully's specific whistle and not generalize it to similar stimuli such as the owner's whistle.
Step-by-step explanation:
The operant principle causing difficulty in training the dog to respond to the owner's whistle is stimulus discrimination. Stimulus discrimination occurs when an organism learns to respond differently to various stimuli that are similar, demonstrating the conditioned response only to the conditioned stimulus. In the case of Mrs. Ridcully's dog training, the dog has learned to associate the specific sound of Mrs. Ridcully's whistle (conditioned stimulus) with the command to sit, but does not generalize this response to the owner's whistle because it does not exactly match the conditioned stimulus. This distinction is crucial in both classical and operant conditioning for shaping behavior, as it helps the organism learn to respond only to specific cues that predict certain outcomes.