Final answer:
The recommended position for postural drainage of the apical lung segment in an adult is left side-lying.
Step-by-step explanation:
The recommended position for postural drainage of the apical lung segment in an adult is left side-lying. This position allows gravity to assist in the drainage of secretions from the apical region of the lungs. It is important to note that the other positions mentioned in the question, including the supine position, standing position, and the Trendelenburg position, are not recommended for postural drainage of the apical lung segment.