The correct answer is breathing humified air.
The humified air assists in liquefying the respiratory discharging, making them easier to raise and expectorate.
The postural drainage may be beneficial for respiratory hygiene but will not influence the nature of secretions. Vibration and percussion of the chest wall may be beneficial for respiratory hygiene but will not influence the nature of secretions.
Deep breathing and coughing exercises may be beneficial for respiratory hygiene but will not influence the nature of secretions.