Passenger Service Units are primarily used for what purpose?

Passenger Service Units (PSUs) are primarily designed to enhance passenger hydration and comfort during a flight. These units are typically located overhead and contain essential features such as reading lights, air vents for ventilation, and controls for individual passenger needs, including oxygen masks in case of an emergency. The focus of PSUs is on providing a more pleasant and comfortable environment for travelers, allowing them to manage their immediate comfort while on board, such as adjusting airflow and lighting to their preference. The other options do not directly relate to the primary function of PSUs, which is why they are not correct in this context.