Final answer:
The priority action for the nurse is to stay with the client when flashbacks occur.
Step-by-step explanation:
The priority action for the nurse to take when caring for a client with posttraumatic stress disorder (PTSD) after military deployment is to stay with the client when flashbacks occur. This is because during a flashback, the client may experience distressing and vivid memories of the traumatic event, which can lead to a sense of fear, panic, or disorientation. By remaining with the client and providing reassurance and support, the nurse can help create a sense of safety and facilitate the client's grounding in the present moment.