Final answer:
The correct technique for fitting a one-way valve mask is to cover both the nose and mouth to ensure effective treatment of sleep apnea with CPAP devices.
Step-by-step explanation:
The recommended technique for fitting a one-way valve mask, such as those used in Continuous Positive Airway Pressure (CPAP) devices for the treatment of sleep apnea, is to cover both the nose and mouth. This ensures that the therapy is effective, as the device needs to provide a constant flow of air to maintain open airways. Thus, the correct answer to the question is: a. Covering both nose and mouth.