Final answer:
The most appropriate intervention is to notify the surgeon.
Step-by-step explanation:
The most appropriate intervention for a client who has just returned to the surgical unit after an open cholecystectomy and has an abdominal dressing saturated with sanguineous drainage is to notify the surgeon. Sanguineous drainage indicates the presence of fresh blood, which could be a sign of bleeding or a complication from the surgery. It is important to inform the surgeon so that they can assess the situation and provide appropriate treatment.