Final answer:
The most important nursing action is to notify the healthcare provider immediately.
Step-by-step explanation:
The most important nursing action in this case is to d) Notify the healthcare provider immediately. Since the client has a closed head injury and is experiencing vomiting, dizziness, and headache, it could indicate a worsening condition such as increased intracranial pressure or a possible concussion. Promptly notifying the healthcare provider is essential for further evaluation and appropriate treatment.