Final answer:
The most appropriate PEEP level for thoracic surgery patients on ventilatory support is 8 cm H2O PEEP.
Step-by-step explanation:
The most appropriate positive end-expiratory pressure (PEEP) level for thoracic surgery patients placed on ventilatory support to prevent atelectasis and improve gas exchange is 8 cm H2O PEEP (Option C). PEEP is a technique that keeps the airways open and helps maintain lung volume during mechanical ventilation. It improves gas exchange by preventing collapse of the alveoli and maintaining a positive pressure at the end of expiration.