Final answer:
The anterior portion of the maxillary impression tray should be seated in the mouth first.
Step-by-step explanation:
The portion of the maxillary impression tray that should be seated in the mouth first is the anterior portion. The anterior portion includes the incisors and the canines, which are the front teeth used for biting and tearing food. Seating the anterior portion of the tray first allows for better alignment and stability during the impression process.