Based on the symptoms and history provided, the most appropriate diagnosis for the client appears to be D. posttraumatic stress disorder (PTSD).
PTSD is a mental health disorder that can develop in people who have experienced or witnessed a traumatic event, such as a violent attack, serious accident, or military combat. Symptoms of PTSD can include flashbacks or nightmares about the traumatic event, avoiding reminders of the event, negative changes in thoughts and mood, and heightened feelings of anxiety and/or hypervigilance.
In this case, the client's reported symptoms of high anxiety, vivid flashbacks, emotional numbing, difficulty concentrating, and insomnia all suggest the possibility of PTSD. Additionally, the history of the client having been attacked by an angry co-worker provides a clear traumatic event that may have led to the development of PTSD.
It is important to note that only a licensed mental health professional can provide a formal diagnosis of PTSD or any other mental health condition after a thorough evaluation.