Final answer:
Suction the airway for up to 15 seconds.
Step-by-step explanation:
When ventilating an apneic adult and noticing that their stomach is becoming distended, the correct action is to suction the airway for up to 15 seconds. This is an important step to remove any potential obstructions in the airway that could cause the distension. It is crucial to ensure a clear and unobstructed airway to prevent further complications.