Final answer:
Dr. Johnson's assumption about Karen's homework habits exemplifies the fundamental attribution error, where internal factors are overestimated and external situations are underestimated.
Step-by-step explanation:
Dr. Johnson's thinking best exemplifies fundamental attribution error. This psychological concept occurs when an individual attributes someone else's behavior primarily to their character or personality, while underestimating the influence of situational factors. In this case, Dr. Johnson attributes Karen's lack of homework completion to laziness (a personal trait) rather than recognizing the potential situational factor that Karen is working 40 hours a week. The fundamental attribution error often leads observers to overestimate the role of internal factors and underestimate the impact of external, situational influences when evaluating others' behaviors.