Final answer:
The sentence 'Dr. Hibbert yawns widely before breakfast' is an example of active voice as the subject, Dr. Hibbert, performs the action of yawning.
Step-by-step explanation:
The sentence 'Dr. Hibbert yawns widely before breakfast' is in the active voice. In an active voice sentence, the subject of the sentence performs the action. Here, 'Dr. Hibbert' is the subject who is doing the action of yawning. Therefore, it's active because the subject, Dr. Hibbert, is actively doing the yawning action, and the yawning is not something happening to him which would be indicated by the passive voice.