Final answer:
The nurse should call the physician immediately when caring for a client with a cerebral aneurysm who reports a severe headache.
Step-by-step explanation:
The correct action for the nurse to perform when caring for a client with a cerebral aneurysm who reports a severe headache is to call the physician immediately. Severe headaches in clients with cerebral aneurysms can be a sign of a ruptured aneurysm, which is a medical emergency. By calling the physician immediately, the nurse ensures that the client receives appropriate medical intervention and prompt evaluation.