Final answer:
Adjustment disorder is considered the least severe of the listed disorders, as it stems from everyday stressors and tends to be more short-lived than chronic conditions such as PTSD.
Step-by-step explanation:
The least severe of the stress-related disorders listed is b. adjustment disorder. Unlike post-traumatic stress disorder (PTSD), which is a chronic reaction to severe psychological trauma, adjustment disorder is a stress-related condition that entails difficulty coping with or adjusting to a specific life change or stressor. It tends to be less severe because it usually arises from everyday stressors, rather than traumatic events, and the symptoms typically resolve within six months if the stressor is removed or if the individual learns to adapt to the stressor.