Final answer:
To enhance the psychosocial development of a hospitalized teen with cystic fibrosis, the most beneficial action is to encourage visits from friends, supporting vital social interaction and emotional well-being.
Step-by-step explanation:
The action that will most enhance the psychosocial development of a 16-year-old male hospitalized for cystic fibrosis is encouraging the teen's friends to visit him in the hospital. This approach addresses the need for social interaction and support from peers, which is critically important during adolescence. While other actions such as faxing the teen's teacher for homework or telling the teen he is free to use his phone can also be beneficial, direct engagement with friends can significantly improve the patient’s emotional well-being and help maintain his social connections during the hospitalization period.