Final answer:
When a newborn is undergoing phototherapy for physiological jaundice, the most crucial action is to cover their eyes to safeguard them from the ultraviolet light.
Step-by-step explanation:
The most important action for an LPN (Licensed Practical Nurse) to take when a newborn is receiving phototherapy for physiological jaundice is to cover the newborn's eyes during phototherapy. Covering the eyes protects them from potential damage caused by the ultraviolet light used in phototherapy. While increasing the newborn's fluid intake may be beneficial for general health, especially considering the immaturity of the neonatal kidneys, it is not directly related to the administration of phototherapy. Additionally, applying sunscreen to the newborn's skin is not recommended during phototherapy because it could block the therapeutic light waves needed to treat the jaundice. Keeping the newborn in a darkened room is also not pertinent to the phototherapy process itself.