Final answer:
The nurse's priority is to observe the client's behavior for signs of self-harm.
Step-by-step explanation:
The nurse's priority in caring for a client with borderline personality disorder and self-harming behavior is to A. Observe the client's behavior. By closely monitoring the client's actions, the nurse can recognize when self-harm may be imminent and intervene appropriately.
While it is important for the client to verbalize their feelings and emotions (option C) and for the nurse to act as a role model for expression of anger (option D), both of these actions are secondary to observing the client's behavior for signs of self-harm.
A verbal no-harm contract (option B) can also be helpful, but it alone is not the priority. The nurse should focus on continuous observation and intervention to ensure the client's safety.