Final answer:
When water is low in a humidifier, one should refill it with sterile water to ensure proper functioning and adequate humidity levels.
Step-by-step explanation:
If water is low in a bubble humidifier, the correct action to take is to refill with sterile water. It is important to maintain the appropriate water level in a humidifier to ensure that it functions properly and delivers the right level of humidity into the oxygen being supplied. Shaking the humidifier vigorously, increasing the oxygen flow, or increasing the temperature would not be appropriate actions and could potentially cause damage to the humidifier or adversely affect its performance.