Final answer:
The nurse should take action if hands are unclean before entering the room of a client with reduced immunity.
Step-by-step explanation:
The observation that requires further action by the nurse is b) Unclean hands before entering the room. Since the client has reduced immunity, it is crucial for the nurse to maintain strict hygiene protocols to prevent any potential infections. Unclean hands can introduce harmful bacteria or viruses into the client's environment and pose a risk to their health.